The cascaded H-bridge multilevel inverter has wide application prospects in high-voltage and large- capacity photovoltaic grid-connected power generation system owing to the advantages of low loading stress of the switch tube and low harmonic content of grid-connected current. The cascaded H-bridge multilevel inverter grid-connected photovoltaic power generation has some problems, such as uneven light intensity and tempera- ture, aging of photovoltaic panels or being shaded, which may lead to unbalanced output power of each phase of the H-bridge inverter, resulting in unbalanced grid-connected current. In this paper, a zero-sequence injection method based on DC/DC converter for three-phase cascaded H-bridge photovoltaic grid-connected system was proposed. The required zero-sequence component was obtained by subtracting the positive sequence envelope voltage from the specific square wave, so that the grid-connected current can still be in phase with the grid volt- age under the condition of unbalanced input power. The simulation results in MATLAB / Simulink environment show that the harmonic content of grid-connected current meets the IEEE Std. 929-2000 standard and can real- ize the operation of unit power factor.