Based on reliability optimization design theory and sub step optimization method

to make the overall volume of the reducer and single stage planetary gear train（PGT） smallest

the mathematic model of transmission ratio distribution and single stage’s parameters are established

respectively.The complex design optimization problem is solved by MatLab optimization toolbox.The optimal design results showed that

in the case of maintaining strict requirements and ensuring high reliability

the volume of the yaw reducer decreased by about 3.5% comparing with the original design

which supplies an effective approach to further developing the reliability optimization design of multi-stage planetary transmission systems.