The best time to go to China is from March to May and from September to October. Though, if you are well prepared, you can travel there any time of year. From November to February it can be very, very cold in the north and the interior. And from June to August it can be very hot, with heavy, and sometimes continuous rainfall.
Here you can check the average temperatures in China as well as monthly rainfall and other weather data.
Weather in China (climate):
Since it’s practically an entire continent, China has an immense variety of climates: from arctic in the north to tropical in the south. In short, the north and northeast (Beijing) have a dry climate, with cool winters and very hot summers. The south is tropical, with short winters and warm, rainy summers. Temperatures in the west are milder year round (except in Tibet, which has a classic high mountain climate). See weather forecast in China for the next 15 days.