Aiming at the complexity of state detection and fault diagnosis of spacecraft power system，a non-invasive real-time monitoring method of power electronic converter is proposed. Firstly，the digital model of the physical model is built. Secondly，the output voltage collected from the physical model is input into the digital model，and the objective function is constructed according to the output voltage and ripple of the physical model and the digital model. The component parameters of the digital model are updated by using the Particle Swarm Optimization (PSO) algorithm，and the fault diagnosis of the physical model is realized. The experimental results show that the proposed method has short time for fault diagnosis and high average accuracy of parameter identification，which can realize real-time monitoring of converter.