A hybrid automata model of bi-directional DC/DC converter is proposed to solve the problems of high frequency dynamic, large amount of calculation and hybrid characteristics of plug-in hybrid electric vehicle (PHEV) hybrid power system. The working principle of PHEV’s hybrid power system is analyzed, the models of each working mode are established based on Lagrangian mechanics, then, based on the theory of hybrid automata and combining with discretization, a hybrid automata model of bi-directional DC/DC converter is established. Simulation experiments are used to verify it. The simulation results show that the hybrid automata model of bi-directional DC/DC converter has the advantages of moderate calculation, high tracking accuracy, it fully describes the various working modes of the system, and the CCM mode and DCM mode are considered, which is in line with the practical application.