In recent years, with the increasing penetration rate for gird integration of new energy generation, the loads are more diversified and nonlinear on the user side, resluting in the decline of power quality. Aiming at the problems of unbalanced output voltage, low-voltage DC voltage oscillation, and harmonic injection of distribution network under unbalanced load, based on the tertiary topology of power electronic transformer with T-type three-level output stage using SVPWM modulation and neutral point potential balance technology.It can ensure three-phase voltage balance or constant DC voltage and realize stable operation of distribution network. Simulation analysis and verification results show that this method is feasible and effective, and can effectively improve the power quality of distribution networks under unbalanced load.