To reduce nodal voltage deviation in distribution system is basic requirement for an active distribution system (ADS) operation. In order to control nodal voltage, power electronic transformer (PET) is proposed. With the help of PWM (pulse width moderation) control at the primary side and the secondary side respectively, power flow at the both side of PET is regulated. Therefore, the optimal passive flow model of an active distribution system is built, which is composed of distributed generators, storage devices and power electronic transformers. Then particle swarm optimization (PSO) is adopted to solve the problem. Simulation results reveal that power loss in the active distribution network is less than that in the same network with on-load tap changer (OLTC) transformer. Meanwhile, the relative error of nodal voltage is less than the latter, which demonstrate that power electronic transformer applied to the active distribution system could improve the operation level with its flexible regulation ability of passive power.