To address the problem of high peak high frequency common-mode voltage due to high frequency switching action in conventional three-phase inverters. The common mode voltage under space vector pulse width modulation (SVPWM) and tri-state space pulse width modulation (TSPWM) of motor drive system is firstly compared and analyzed, and on this basis, the relationship between the modulating waveform of discontinuous pulse width modulation (DPWM1) and the modulating waveform of TSPWM is investigated, and the method of realizing TSPWM modulation based on carrier wave is given. Then, the relationship between positive and negative polarity carriers is studied, and a way of implementing negative polarity carriers in engineering is given. Finally, the TSPWM modulation implementation proposed is verified by simulation and experiment. The results show that the modulating waveform of TSPWM can be implemented in the same way as the modulating waveform of DPWM1, and the pulse width modulation (PWM) signal under a negative polarity carrier can be generated by a positive polarity carrier instead.