In this paper, the contact stress of reducer gears is simulated and analyzed, and the assembly error is considered. Application of three-dimensional modeling software SolidWorks helical gear model is set up, and carried out in accordance with the working condition of two kinds of error assembly, including the backlash error and axis parallelism error, and the finite element analysis software ANSYS, the contact surface and tooth root, the contact stress of the online statics and dynamics analysis, it is concluded that the stress under two kinds of error and change situation, the size of the summary analysis of the influence on gear transmission, in the future has reference and guiding significance to the actual production of gear assembly. Combining the powerful modeling function of CAD software with CAE, it embodies the integration of CAD and CAE technology.