Abstract: Electric vehicles need to output multiple different levels of voltage and require a switching power supply to complete power conversion. Among them, Buck type switching power supply is the most widely used. However, switch mode power supplies are nonlinear converters, and different switch states have different equivalent circuits, making it difficult to use classical control theory for output voltage regulation. Therefore, this article first adopts the state space averaging method to construct a new small signal mathematical model of Buck type switching power supply. Then, by combining the amplitude frequency and phase frequency characteristic curves, the controller parameters are designed. The designed PI controller parameters not only enable the system to have good steady-state and dynamic performance, but also have disturbance suppression performance for input voltage and load current changes. Finally, the proposed method was simulated and experimentally validated, and the results consistently demonstrated its feasibility and effectiveness.