The torque density of five-phase permanent magnet synchronous motor (PMSM) can be increased by injecting the third harmonic current. Besides the third harmonic, weakening the other spatial harmonics generated by the permanent magnet can reduce the torque ripple, vibration and noise of the motor. Therefore, an optimization design method of rotor shape is proposed for the third harmonic injection five-phase interior permanent magnet synchronous motor (IPMSM). The analytical expression of the rotor core shape is deduced theoretically, and the finite element model is built according to the motor parameters. The air-gap flux density harmonic content and torque ripple of the optimized motor model are obtained. The motor models before and after optimization are compared in simulation, and the simulation results are consistent with the theoretical analysis, proving the significant optimization effect of air-gap magnetic field.