Abstract: As an important implementation form of energy router, solid-state transformer（SST） has broad application prospects in AC/DC distribution networks, energy distribution and control of microgrids, etc. In order to improve the operation stability of the SST and overcome the problem of “multi-device” state monitoring and “multi-switch” open-circuit fault location of the SST, the digital twin method of small SST state monitoring and single-tube open circuit fault diagnosis is proposed by using the high fidelity and interoperability of digital twins. This method uses electromagnetic transient program to build digital twins that can operate under open-circuit fault conditions. Combined with the data collected by the physical system, the quantum genetic algorithm is used to realize the parameter synchronization and state monitoring of the key components of the digital twins. By comparing the electrical quantities of digital twins and physical systems, a single-tube open-circuit fault diagnosis algorithm is proposed to accurately locate the open-circuit faults of switching device transistors and reverse parallel diodes. The simulation results of the case show that the proposed method can realize accurate state monitoring and fast open-circuit fault diagnosis for devices.