In order to improve the assembly quality of the motor and reduce the total cost,it is necessary to optimize the design of the tolerances of the key components that form the air gap between the motor stator and the rotor. A multi-objective and multi-constrained mathematical model for radial tolerance optimization of a motor assembly was established with processing cost and mass loss cost as objective functions,tolerance values of key components as design variables,and assembly functions and economic processing capabilities as constraints. The model was solved based on the improved genetic algorithm,and the key component tolerances under the lowest total were obtained. Through CETOL 6σ simulation verification,the results show that the passing rate of motor assembly is increased by about 55. 7%,and the total cost is reduced by about 45. 2% with the optimized design tolerance.