Does china use the metric system of length? 

China does use the International System of Units (SI) and it was made law to do so officially on 16th February 1929. 

All road speed signs in China are displayed in km/h, for example 30km/h or 60km/h. Distance to next town are all displayed in km, for example 10km or 150km. 

Chinese people use meters and centimetres but in some trades people may use a traditional system of inches (cun) and feet (chi). 

A cun, or one Chinese inch, is not the same length as an English/American inch, and a chi, or one Chinese foot, is not the same length as an English/American foot. 

For comparison: 

There are ten Chinese inches in one Chinese foot. 

There are twelve English/American inches in one English/American foot. 

You will find many Chinese tape measures with both mm, cm and meters on one side and cun on the other side.

Summary