In order to reduce the output torque ripple of the built-in PMSM, a rotor optimization design method is studied, which uses a broken line permanent magnet, a segmented arc structure and an angle groove at the arc connection point. The Taguchi method is used to determine the best combination of key parameters, and the performance of the motor is compared with the traditional "one" structure. Firstly, the different rotor structures and performance characteristics of the built-in permanent magnet synchronous motor are compared and analyzed. The equivalent magnetic circuit model of the no-load motor is established by using the equivalent magnetic circuit method, and the no-load air gap magnetic density is calculated and verified by the finite element simulation. Then a two-dimensional finite element model of 3-phase, 4-pole, 24 slot built-in permanent magnet synchronous motor is established. On this basis, the motor is optimized, and the speed range of the optimized motor is expanded by 50% compared with that before optimization. Finally, the torque of the motor is optimized.