The cascade module of power electronic transfoemer (PET）has the problem of DC side voltage unbalance, which affects the stable operation of the system seriously. For power electronic transfoemer, the problem of DC side voltage unbalance in the cascaded H bridge rectifier is studied in this paper. The mathematical model of cascaded Hbridge rectifier is established, the reasons of voltage imbalance between cascaded Hbridge modules are analyzed, and the control strategy of voltage power balance is put forward, based on the reconstruction technique about modulation wave vector. By reconstructing the modulation wave vector, the balance of DC side voltage is achieved. The equal distribution of active power toward each H bridge is ensured, which makes the reactive power flowing through the H bridge to be allocated according to the requirement. Finally, the correctness and effectiveness of the method is verified by simulation in MATLAB/Simulink environment.