I have been flying into and out of China for well over 20 years. I do so almost every time I strap into an aircraft. Requesting weather deviation is the same as anywhere else in the world. Whether you want 5 miles or 50 miles deviation, just tell them what you need and you will usually get it.
However, it is not unusual to have deviation refused due to military activity or proximity to sensitive airspace. The controllers will usually offer an alternative deviation on the other side of the airway. If you have no alternative but to deviate against the controller’s refusal then just make a PAN call stating that you are deviating due thunderstorm. You will then hear the supervisor come over the top to approve the deviation and to ask you to advise when clear of weather.
I hope this helps. Enjoy your trip to China. It is not the Wild West anymore. There are plenty of frustrating delays (especially in summer) but things run along relatively well considering the huge yearly increases in traffic levels. Just stay flexible, follow instructions and remember that when you want to blow your stack in frustration that you are the only one who is getting stressed up about your situation. So just sit back and call for another coffee and the newspaper.