Abstract: The application of solid state transformer (SST) in new type power system is gradually increasing. Due to its complex topology, numerous nodes and high switching frequency in sub modules, the efficiency of electromagnetic transient simulation for SST is low. At present, the research on the large-step simulation method of SST is inadequate. Therefore, a large-step simulation method of SST based on small-step synthesis in discrete state space is proposed. First, the discrete state space model of small-step modeling and small-step simulation is established. Then, according to the characteristics of the discrete state space equation, the small-step iterative synthesis method is used to construct the discrete state space large-step simulation model to realize small-step modeling and large-step simulation. Finally, a quadratic equivalent method for the large-step simulation model is proposed, which reduces the coefficient matrix dimension of the overall system modeling and reduces computational complexity. The results show that the proposed method can not only reduce the numerical integration error and power electronic switch action error, achieve accurate simulation of SST converter system at 100 kHz switching frequency, but also significantly improve the simulation efficiency of SST.