Abstract: Based on a synchronous machine in an AC—DC—AC variable frequency speed-regulating drive system, a new power supply method of phase-shifting combination is proposed by a detailed study on the impact of power grid harmonics. With regard to the auxiliary motor drive system of a hot-rolling mill, the grid side harmonic rate and waveform distortion rate are quantitatively analyzed when the parameters of the power load, LC filter, pulse-width modulation circuit and transformer change. Simulation results show that the power supply method with the reasonable parameters3 design of the drive system can drastically reduce the grid side harmonic interference that the total harmonic distortion is suppressed within 4.0%. This study provides an effective method for the design of a factory drive system and a reactive power compensation system as well as the fault diagnosis of an electric drive system.