Power electronic transformer (PET) is the key equipment to realize the power conversion and routing of AC/DC hybrid distribution network, which can greatly enhance the flexibility and controllability of power grid. At present, PET with cascaded H-bridge (CHB) and series resonant dual active bridge (SRDAB) is widely used because of its modular structure and high efficiency. In this paper, a 1.5 MW 10 kV AC-750 V DC PET uesd in practical engineering is analyzed. Firstly, the different switching transient characteristics under the forward and reverse power flow direction are described, and the reasons for the difference are analyzed. Secondly, the current on the rectifier side of SRDAB only flows through the diode. Therefore, the switching loss of SRDAB can be reduced by locking the power semiconductor device on the rectifier side, so as to improve the efficiency of PET. Finally, the experimental test on the PET project site proves the correctness of the loss analysis and the effectiveness of the efficiency optimization method. Using the proposed method, the PET operation efficiency can be improved by about 0.2%.