By using epicyclic train to close differential gear train,it makes this kind of gear train with high efficiency,large transmission power,wide transmission and compact structure,etc.The form of gear transmission is adopted in the epicyclic train.A Gear/Chain mixed transmission planetary gear reducer is designed by using gear/chain instead of parts of gear to realize the function of epicyclic train.On the basis of the theoretical calculation method of epicyclic train transmission ratio,the calculation formula of gear train transmission ratio is derived.According to the method of graphic to draw the power flow diagram of gear/chain mixed transmission for planetary gear reducer.The power flow analysis of the transmission system is carried out through these diagrams,and the calculation of the transmission efficiency is carried out.