The word power means a couple of things, depending on how you use it. According to Merriam Webster two of the definitions of power are as follows:
legal or official authority, capacity, or right
as well as ...
possession of control, authority, or influence over others
In terms of your sentence, power here relates to political power, namely the "legal or official authority" of a government (in this case the Chinese Communist Party) in that they are in "possession of control" of China and "have authority and influence over others", namely the Chinese people.
Adding the word in, in front of the word power just tells to us who held (or is currently holding) that political power.