With the large amount of distributed energy entering power grid and the increase of DC load, multi-terminal AC/DC distribution network becomes the future development trend, and multi-port power electronic transformer becomes the core equipment. In the multi-port power electronic transformer with series connection H-bridge (CHB)structure, there are a lot of full bridge modules and bidirectional active bridge (DAB)modules. At the same time, many factors such as high switching frequency make it very difficult to simulate switching devices model of the multi-port power electronic transformer. Especially in the case of AC/DC distribution network with multiple power electronic transformers interconnected, the system simulation is almost impossible to be completed. In order to solve the above problems, a transient average model is proposed. By modeling and simplifying the multi-port power electronic transformer and simulating it with unlocking model in steady state, research and simulation on system level verification such as coordinated control of multi-terminal system, can be supported. In case of fault, the system fault simulation is completed by switching between unlocking equivalent circuit and locking equivalent circuit, which improves the simulation speed. Finally, the correctness of the proposed model is proved by comparing the simulation results under DC fault.