A modular multilevel converter (MMC) based four-port power electronic transformer (PET) based on is proposed in this paper. Medium-voltage AC(MVAC) and medium-voltage DC(MVDC) ports provided by MMC, adopts the MMC SMs of upper and lower arms with symmetric structure to build the low-voltage port, in each phase, the first submodule of the upper bridge arm and the last submodule of the lower bridge arm are used to build low-voltage DC(LVDC) port, and the last submodule of the upper bridge arm and the first sub-module of the lower bridge arm are used to build low-voltage AC(LVAC) port. They are cascaded with DAB for step-down and power isolation. The working principle and control strategy of the four-port are introduced respectively, a voltage fluctuation compensation control strategy for low-voltage terminal module is proposed to reduce capacitor voltage fluctuation effectively. It has the advantage of using fewer switching devices and transformers (56.8% lower cost compared with the existing traditional PET). The feasibility of the proposed PET and the effectiveness of the overall control strategy are verified by simulation.