Power electronic transformer has the advantages of small volume, light weight and various functions. However, the power semiconductor devices are prone to open circuit under the condition of overvoltage or overcurrent, reducing the reliability of power electronic transformer. In this paper, an improved open-circuit fault diagnosis method is proposed, which is carried out by comparing the voltage of LLC primary and secondary DC bus. Then a fault-tolerant method based on structural reconstruction is presented, which ensures normal operation of the power electronic transformer under open circuit faults. Finally, a 1 kW single-stage matrix power electronic transformer experimental prototype was built for verification. The experimental results verify the correctness of the proposed methods.