Abstract: As an important scheme for grid connection of photovoltaic, wind power and other DC power sources, modular isolated DC/DC converter (MIDC) has received extensive attention. The input parallel output series (IPOS) type single active bridge (SAB) converter is one of the common topologies of MIDC. Due to the high node admittance order, low simulation step size and the existence of uncontrolled rectifier bridge, its electromagnetic transient simulation efficiency is extremely low. This paper proposed an equivalent modeling method of SAB converter by the pre-calculating current zero-crossing. First, the topology and working principle of the SAB converter were analyzed to solve the inductor current expression in different modes. Secondly, the expression of the zero-crossing point of the inductance current was given with the distinguishing conditions of different modes. Then, analogy to the DAB converter, the equivalent model of the IPOS type SAB converter was established. On this basis, Lyapunov method was used to prove the stability of the proposed equivalent model. Finally, the accuracy and acceleration ratio of the proposed model were verified in the PSCAD/EMTDC environment.