The coupling transmission error model of multistage planetary gear is developed according to precision transmission’s requirement from a geometrical perspective，comprehensively considering eccentric error of the components in machining and assembling，deviation of pitch and deviation of tooth thickness. The work describes the stochastic process of the random variables in the model by Monte Carlo methods，which corresponds better with the law of error statistics. The coupling transmission error model of a material example is analyzed and simulated by the proposed method. Furthermore，the most influential error factors are figured out by spectral analysis. Thus，the results show that random error value is proportional to transmission error，and that the randomness of error’s initial phase tends to reduce transmission error.