The present invention provides a Pulse Width Modulation (PWM) method for a Cascaded H-bridge (CHB) converter. Each phase of the converter is provided with n Cascaded H-bridge rectifier circuits, or may also be provided with n Cascaded H-bridge rectifier circuits+1 redundant H-bridge rectifier circuit. The method includes following steps of: S1, generating groups of sinusoidal signals as reference waveforms, and generating n carrier signals having sequentially decreasing levels and equal amplitudes to correspond to the H-bridge rectifier circuit at the first to the nth levels, where the levels of the n carrier signals are cascaded to fill the voltage amplitude of a unipolar half cycle of the reference waveform; and S2, determining PWM signals for controlling power transistors of the corresponding H-bridge rectifier circuits based on each reference waveform and each carrier signal. According to the present invention, the CHB converter allows the MV grid to be directly coupled with a LV side without a conventional transformer, which reduces the heat loss and balances the distribution of heat loss between the power transistors and between the H-bridges at all levels, thus prolonging the service life.