In order to improve the efficiency of the frameless permanent magnet synchronous motor for robot joints and reduce the cogging torque and torque ripple,the analytical formula of the motor cogging torque was obtained by the energy method.Four important parameters,such as air gap length,permanent magnet thickness,permanent magnet pole arc coefficient,and stator slot width,which affected the torque performance of the motor were determined.The influence of each parameter on the cogging torque was analyzed by the finite element method.Based on the response surface method and multi-objective genetic algorithm,the response surface regression model was established to improve the motor efficiency and reduce the cogging torque,and the Pareto front of the permanent magnet synchronous motor was obtained.Fianlly,the optimal solution was selected according to the constraints and design objectives,and the initial scheme and the optimized scheme were compared and analyzed by finite element analysis.The results show that the multi-objective optimization method can be used to design the target motor with both high efficiency and low cogging torque.