This passage has analysed the main factors which influence the transmission precision of planetary gear reducer.The researched backlash includes manufacture error of the main accessory in planetary gear reducer,assembled error,transfiguration produced by temperature and elastic transfiguration ect.This passage focuses on quantitative analysis to math relation between aperture and backlash.At last,this passage brings forward how to minish the backlash in the accessory choosing,structure design ect.