What is your destination? Shenzhen? The weather in Shenzhen is very pleasant all year round. It is not very cold in December.

It depends what you find to be cold. For a European they would likely not find it cold but Indonesians may well feel it cold. December is usually starting to get to the coldest time in Shenzhen which is January. Top temperatures will be somewhere between 15C and 22C. Low temperatures could be as low as 8C but unlikely to be any lower. I lived in Shenzhen for four years and found the weather wasn't too bad but coming from Melbourne Australia it was temperatures I was used to.