Abstract: The third harmonic compensation strategy（THCS） can effectively solve the power imbalance problem of cascaded H-bridge（CHB） photovoltaic（PV） grid-connected inverter. Compared to other methods, the THCS does not reduce the power generation, and ensures that the system is able to operate with unit power factor and the fluctuation of DC side voltage is also relatively small. However, the proposed THCS has some problems in eliminating the third harmonic of the grid current, it can’t eliminate the third harmonic effectively. In this paper, an optimized THCS for single-phase CHB inverter has been proposed, it eliminates the third harmonic in the grid current through closed-loop control, and the suppression effect is greatly improved compared with the traditional THCS. Simulation and experimental results all verify the effectiveness of the proposed strategy.