Its immense size and enormous diversity of environments and climates gives China an enormous advantage: you can visit it all year round. Yes, but be careful: every season has its good points and each region has periods when it is better to visit it.
Whether you are looking for adventure or a cultural immersion, whether you like lots of heat or the comfort of a parka, whether you prefer lying on the beach with friends, or solo trekking in the rice paddies, China has something for you. So long as you carefully research the climate, festivities, and tourist high season in the area that you want to go to.
All the northern regions of China have a harsh winter, cold, (almost glacial), and very long (from October to April in certain areas!). Some itineraries, especially those which traverse mountains and deserts are very difficult to do...
The rest of the year is much milder, with dryish summers, despite the influence of the monsoon - which can be inconvenient - and scorching temperatures on the Silk Road. So you've got it, the end of spring and the beginning of the fall are the nicest seasons if you want to take a trip to north China.
The best months for going to the central regions are May and October. The temperatures are mild, and the vegetation and colors beautiful. Summer is marked by a heavy monsoon - so to be avoided if you don't like humidity. The winter is milder than in the north except for a few areas that are quite cold.
Scichuan's climate changes according to the altitude : milder but with fog and rain on the plains, drier but cold on the high plateaux and high mountains, but with everywhere influenced by the monsoon in the summer.
Tibet is the most arid region in China and rain is rare. On the other hand, there are very marked changes in temperature. In summer the temperature can reach 30°C on really sunny days, but that's also the high tourist season. The end of spring or the beginning of fall are good alternatives.
Although the climate is less harsh than in the rest of the country, it is much more humid. The further south you go, the more marked is the rainy season, with a summer monsoon that can become suffocating. Some people, therefore, prefer to wait for the fall. And the winters are more or less mild.