In order to improve the deficiencies of traditional power electronic transformer (PET) which are not suitable for high-voltage high-power power system, modular multilevel converter (MMC) is applied to the input stage of PET to increase the voltage level and power level of PET. The output stage of PET is composed of a three-phase full-bridge inverter, and a permanent magnet synchronous motor (PMSM) is connected to the output stage of PET, and the passivity-based control (PBC) method based on port controlled Hamiltonian with dissipation (PCHD) model is proposed to control the output stage of PET, which can be further applied to train traction system and wind power generation system. Finally, the simulation experiment is carried out on the simulation platform, and four different working conditions are simulated, including no-load operation, constant speed operation, variable speed operation and load torque mutation, the results show that the proposed control strategy has the characteristics of strong robustness and good dynamic performance.