China is a vast country with wide-ranging climatic conditions, so a decision about the best time to visit should be based on the regions you plan to tour and the kind of weather you enjoy.

 Autumn
Normally, the most comfortable season of the year is early autumn (September to early October). During that period, temperatures are reasonable throughout China (about 50-72F) with a limited amount of rain. September for example is the only month in the year when the ancient and valuable paintings of the Beijing Palace Museum are displayed due to proper climate conditions (low humidity and proper temperature).

 Spring
Spring can also be delightful with the average temperatures roughly the same as in autumn (about 50-72F). The best way to deal with weather unpredictability is to wear layered clothing that will make you comfortable in both chilly and warm weather.

 Summer
Summer (from June to the end of August) can be extremely hot with temperatures well above 72F, especially in the famous 'four furnaces' of China: Wuhan, Tianjin, Chongqing and Nanchang.
Summer is also a rainy season, so travelers should not forget umbrellas, light raincoats and rubber/plastic shoes.

 Winter
Winter can be incredibly cold especially in the north. Off season travel can also offer its rewards. For example, the Harbin Winter Ice Lantern Festival is quite charming.

Before deciding when to take a tour, it is worth checking carefully the China weather conditions of each city on the itinerary. Needless to say that the most attractive seasons to visit are also the most crowded. Regardless of the time of the year, a visitor will always be rewarded with charming scenery throughout China.

High Season vs Low Season

High seasons in China:
• Labor Day (May 1, 3 days duration)
• National Day (October 1, 7 days duration)
• University Holidays: Summer holiday (June-September) and Winter holiday (January-February)

Many people plan a trip during high seasons. However it is not always the best choice. During high seasons tickets for trains, planes, as well as accommodations at hotels are hard to obtain, as great numbers of people travel during these times. It is highly recommended to join an organized tour if you must travel during high seasons, as doing so will certainly relieve you from the difficulties of obtaining tickets.

Compared with high seasons, low seasona couldn't be a better choice for traveling. They offer you the following merits:
• Saving money. In low seasons, the related costs will be much lower than during the peak seasons. Entrance fees, travel ticket prices, and the goods sold in the places of interest will all cost less.
• Saving time. Low seasons offer you a more tranquil environment rather than a noisy, crowded place. It may take several hours to take a photo during peak time, while in low seasons you can enjoy yourself in the beautiful scenery without any interference.

Festival Preference

China is a great country with numerous nationalities and each nationality has their own festivals. Should you be a Chinese culture lover, come to China during the time the festival you wish to see is happening.  
Nearly each Chinese festival has its own legend and history. Please explore more details about Chinese festivals and determine your trip itinerary based on your favorite ones.

Related link:
yearly climate of China cities