It's a good question to ask because the quality of your China experience can be largely contingent on what the weather is like at the time you travel there.
China is such a huge, vast area that all weather is represented -from hot and cold extremes to mild and tropical, it's all in there.
The best time to visit anywhere in China is during the Spring (April-May) and Autumn (September-November), before it gets too hot or cold.
|One of my favorite things about Beijing is it's tremendous thunderstorms|
|Dust Storm in Beijing (photo courtesy of CNN)|
the winter to not so cold the rest of the year. Weather in the South is hot and humid in the Summer and cool and wet in the Winter. Eastern and southeastern China experiences very little change between the seasons and have monsoons. Western China is mostly desert and plateaus and has long, hot, dry Summers and cold Winters.
Anytime can be a good time to travel to China as long as you're prepared!
Here are some additional resources and links I have read about the varying climates in China:
“It’s Just a Storm”: The Ebb and Flow of Yangtze River Delta Weather
Climate in Western & Eastern China
China Weather: iExplore
Voyages Maps: Chinese Climate
|China climate map (Voyages maps)|