n a single-phase grid-connected inverter and three-phase grid-connected inverter with asymmetric grid voltage, the bus voltage on the DC side contains an obvious double power frequency ripple component. Affected by this ripple component, the AC output side of grid-connected inverter contains an obvious third harmonic, which affects the output power quality of the inverter. To solve the above problems, the output harmonic characteristics of grid-connected inverters are analyzed using a dual Fourier transform and switching function method. An improved pulse width modulation method is proposed to suppress the third harmonic output of single-phase and three-phase inverters. This method corrects the modulation wave according to the DC bus voltage, and does not need to extract the ripple component information of the DC bus voltage. The algorithm is low in complexity and easy to implement. The feasibility of the new modulation method is proved by the switching function method. The control strategies of single-phase and three-phase photovoltaic inverters based on this modulation method are given. Simulation results show that the proposed modulation method can significantly reduce the output third harmonic components of grid-connected inverters with secondary ripple in the DC bus voltage. This work is supported by the Key Science and Technology Project of Henan Province (No. 182102210333).