For people who would normally require to obtain a visa in advance there are certain emergency situations which will permit them to get a visa on arrival.  These will always require an invitation from an accredited Chinese organisation which explains the emergency, (mostly related to business purposes which from the Chinese perspective it is important that you are admitted without delay) or you must yourself be in a real emergency situation such as a passenger in a plane or ship in mechanical difficulty.

Aliens who hold ordinary passports issued by countries which have diplomatic relations or official trade links with China as well as letter(s) or telegram(s) from authorized units in China, may, under any of the following circumstances where they must necessarily rush to China but have no time to apply for a visa, may apply to the visa-granting departments at entry ports as authorized by the Ministry of Public Security. The Rules on Visa Issued at Entry Ports are not applicable to the nationals of United States and France.

1. They are invited at the last moment by a Chinese host to come to China for a trade fair.
2. They are invited to come to China to submit a tender or to sign a formal economic or trade contract.
3. They come to China by appointment to supervise the inspection of import and export commodities or to participate in a check-and-accept operation in accordance with contracts.
4. They are invited to participate in the installation of equipment or in the emergency repair of engineering projects.
5. They come to China at the request of the Chinese side to solve a problem of claims.
6. They are invited to come to provide technical advisory services.
7. They come to China due to a last-minute change in the composition of a visiting group and with consent of the Chinese side after the visas are granted.
8. They come to China to see patients in critical conditions, or to undertake funeral matters.
9. Owing to force majeure, transit visa holders cannot leave the country within twenty-four hours by taking the original place or by taking other means of transport.
10. Other invited guests who really have no time for visa application to the aforesaid China's resident agencies abroad, and who hold letter(s) or telegram(s) from competent authorities who give the consent for the invited guests to apply for the visa at the designated entry ports.

The visa-granting agencies at entry ports shall not accept and handle visa applications filled out by people who do not come under the aforesaid circumstances.

Some accredited International Travel Agencies incorporated under Chinese laws may apply for group-tour visa at entry ports with invitation letter issued by provincial office of foreign affairs.

Visa-issuing departments at port of entry, as authorized by the Ministry of Public Security, are stationed at the following ports: Beijing, Shanghai, Tianjin, Chongqing, Dalian, Fuzhou, Xiamen, Xi'an, Guilin, Hangzhou, Kunming, Guangzhou (Baiyun Airport), Shenzhen (Luohu, Shekou, Huanggang), Zhuhai (Gongbei), Haikou, Sanya, Jinan, Qingdao, Yantai, Weihai, Chengdu, Nanjing, Changchun, Huichun, Hekou, Mohe.

There is also provision for a short stay visa on arrival for some nationalities for persons who only want to visit a Special Economic Zone such as Shenzhen, Zhuhai, Xiamen or Shantou.  Most travellers would find this not an option if they intend flying there.  See Airline Regulations

Shenzhen Visa on Arrival 
The single entry five-day Tourist Visa is available at Luohu (Lo Wu) Port, Huanggang Port, Shekou Port and Fuyong Port if you visit Shenzhen for 5 days at most. There is no visa office at Futian Port although it is a few hundred meters apart from Huanggang Port. They are different land crossings which connect to different points in Lok Ma Chau in Hong Kong. Futian Port connects to Lok Ma Chau MTR Station while Huanggang Port connects to 24-hour Lok Ma Chau Control Point. 

Office Hours of Shenzhen Visa on Arrival Application at the Following Ports:
Luohu Port: 09:00 - 22:30
Huanggang Port: 09:00 - 13:00, 14:30 - 17:00
Shekou Port: 08:45 - 12:30, 14:30 -17:30
Fuyong Port: 08:30 - 17:30

Requirements of Shenzhen Visa on Arrival Application
1. Your valid passport 
2. complete a visa application form when you are at the port
3. take a photo when you are at the port 
4. pay visa fee

Most nationalities of the following countries can easily get a visa on arrival in Shenzhen, such as Australia, Canada, New Zealand, UK, Ireland and Spain. Other nationals are hard to get a visa on arrival if they have never been to China. Passport holders of US and France are not eligible for visa on arrival application and even get fined for arrival without a valid China visa. Currently, the visa fee for most nationals is 168 RMB, but the fee for UK nationals is much higher at a rate of 469 RMB. If Irish nationals are unlucky to meet a border official who doesn't know UK and Ireland are different countries, they will be charged the high visa fee as same as UK nationals. The different visa fees are set on a reciprocal basis. Only RMB is accepted for payment. When you get to the visa office at the border, just complete a visa application form, pay the fee and take a photo, so you don’t need to worry about the application materials. What you need is just a valid passport. The visa will be issued within 10 minutes. 

Zhuhai Visa on Arrival 
Three-day visa on arrival are provided at Gongbei Port if you only visit Zhuhai. Upon arriving at the Gongbei immigration building, you can find the visa office by passing all the duty free shops; it is behind a green door on the right hand side of the immigration building. You shall fill in one visa application form, pay the visa fee and have your photo taken on spot. It takes about 10 minutes to process the visa. It costs 168 RMB for applicants of most countries, but UK nationals have to pay 469 RMB. Hong Kong dollars are not accepted. Only RMB is accepted for payment. Currency can be changed at the China Tourist Services Office back downstairs in the immigration hall. Once the visa has been issued, proceed to the immigration hall to complete the immigration process.   

Hainan Visa on Arrival
Compare to Shenzhen, Zhuhai, Xiamen and Shantou, Hainan enjoys more privileges to issue visa on arrival besides Hainan Visa Exemption. Haikou and Sanya can issue 30 days visa on arrival to all tour groups to Hainan and aliens who go to Hainan for sightseeing, family visit or business. If aliens ask for more than 30 days stay duration for family visit, business, study or work, 3 months China visa on arrival can be issued. They can use the visa to visit other cities within Mainland China and exit Mainland China from all ports open to foreigners. In addition, the visa on arrival is also available for travelers on international cruise ships who disembark in Haikou or Sanya. The international ships are from Hong Kong, Japan, the Netherlands, Germany, France and Portugal.

 China Visa Application Requirements
Requirements for China Visa Application at China Embassy/Consulates in USA
Requirements for China Visa Application at China Embassy/Consulates in UK
Requirements for China Visa Application at China Embassy/Consulates in Canada
Requirements for China Visa Application at China Embassy/Consulates in Australia