Aiming at the poor performance of permanent magnet synchronous motor(PMSM) under low carrier wave ratio synchronous modulation caused by low switching frequency of power devices, a pulse width modulation(PWM) analysis method based on the mathematical model of PMSM is proposed. A comparative study of selective harmonic elimination pulse width modulation(SHEPWM) and central 60 degree modulation is carried out, the current harmonic and torque ripple indexes of the two modulation methods under different carrier wave ratios, as well as the advantages, disadvantages and application scope of the two modulation methods are analyzed. Finally, through simulation and experiment, the application of SHEPWM and central 60 degree modulation is realized on the metro traction system, and the correctness of the theoretical analysis is verified.