Cascaded H-bridge converter is widely used in medium voltage applications, especially in the input stage of power electronic transformer, because of its cascade structure. However, under unbalanced grid conditions, the power fluctuation on AC side will cause the voltage ripple on DC side to increase abruptly, resulting in the current distortion on grid side. Aiming at this problem, a harmonic suppression method for cascaded power electronic transformers under unbalanced conditions is proposed in this paper. The generation mechanism of grid side harmonics is deduced and analyzed, and the coupling mechanism of AC side harmonics and DC side ripple is revealed; Based on this mechanism, an improved quasi-proportional resonant controller is proposed to suppress the grid side harmonics, and the stability of the control system is analyzed; A simulation model is built in PSCAD/EMTDC, and compared with the traditional control strategy, the effectiveness of the proposed harmonic suppression strategy is verified.