Spread over such a vast area, China is subject to the worst extremes in weather, from the bitterly cold to the unbearably hot. The following info is a rough guide to avoid the extremes of temperature.
China lies mainly in the northern temperate zone under the influence of monsoon. From September and October to March and April next year monsoon blows from Siberia and the Mongolian Plateau into China and decreases in force as it goes southward, causing dry and cold winter in the country and a temperature difference of 40 degrees centigrade between the north and the south. Monsoon blows into China from the ocean in summer, bringing with them warm and wet currents, thus in both the north and south of the country of the country, most of the rainfall comes. Great differences in climate are found from region to region owing to China's extensive territory and complex topography.
In northern China, winters fall between December and March and are incredibly cold. From Beijing northwards to Inner Mongolia and Three North-east province Heilongjiang, Jilin and Liaoning, the temperature doesn't rise above 0℃(32℉) and can drop colder to -44℃(-40℉). In Inner Mongolia, you will see the curious phenomenon of sand dunes cover in snow. Spring and Autumn are the best times for visiting the north. Daytime temperatures are in the 20℃ to 30℃(70℉-80℉) range and there is less rain.
In the center of China - in the Yangtze River Valley area (and this includes Shanghai), summer are long, hot and humid. Chongqing, Wuhan and Nanjing have been dubbed 'the three furnace' by the Chinese. You can expect very high temperature anytime between April and October. The southern part of the Yunan-Guizhou Plateau is spring-like in all seasons. Winters are short and cold, with temperatures dipping well below freezing, almost as cold as Beijing. It can also be wet and greatly miserable variations of temperature at any time apart from summer day. So it is impossible to pinpoint an ideal time, but spring and autumn are probably the best time to visit the area.
In the far south, around Canton, the hot, humid period lasts from around April through to September, and temperatures can rise to 38 ℃ as in the north. This is also the rainy season. Typhoons are liable to hit the south-east coast between July and September.There is a short winter from January to March, nowhere near as cold as he north, but temperature statistics don't really indicate just how cold it can get in the south; bring warm clothes. If you flop into China around this time, wearing thongs, shorts and a T-shirt, you'll see what I mean.Auntum and spring can be a good time to visit, with day-time temperatures in the mid-20℃. But it can be miserably wet and cold, with perpetual rain or drizzle, so be prepared.
Try to avoid the Xinjiang region (in the north-west of China) at the height of summer. Industrial Urumqi is dismal at this time (although it's a good time to visit the lake of Heaven in the mountain to the east of the city), and Turpan (which is worse off for being situated in a depression) deserve the title of the 'hottest place in China' with maximums of around 47 ℃.In winter, though, this region is as formidably cold as the rest of northern china. In Urumqi the average temperature in January is around 10 ℃, with minimums down t almost -30℃. Temperatures in Turpan are only slightly more favorable to human existence.
Tibet deserves a special mention since the high altitude creates its own problems. Winters at high altitudes can be endless days and nights of incredibly piercing, dry cold. Summers are warm and dry, because of the thin air, a cloud across the sun will make the temperature drop suddenly and dramatically and you'll always need to carry a jacket with you. For more details, see the section on Tibet.
The warmest regions in winter are Xishuangbanna near the Laotian border, the south coast of China and Hainan Island. In summer, high spots like Emei Shan would be a welcome relief from the heat.