According to the principle of magnetic energy virtual displacement and the characteristics of double Y shifted by 30° winding, the analytical expression of torque in six-phase permanent magnetic synchronous motor (PMSM) is derived, and the impact of harmonic magnetic field on torque is analyzed in this paper yet. On this basis, the torque performance of three-phase and six-phase PMSM with 96 slots 88 poles are simulated, and six-phase motor with the third harmonic current injection is simulated meanwhile. The simulation results show that the analytical expression is correct, and the six-phase double Y shifted by 30° winding PMSM with similar pole and slot number can eliminate the sixth ripple torque in the three-phase one, and the third harmonic current loaded in it can also produce average electromagnetic torque.