Abstract: To resolve the problem that the switch frequency of sliding mode control (SMC) switching power converter with hysteresis modulation method changes with the variation of the input voltage or load, this paper presents a fixed-frequency modulation method that can directly convert a switching function to a fixed-frequency switching control signal. The operation principle of the modulation method for a SMC buck converter is described, and its mixed implementation circuits was given. Simulation results show that the modulation technique reserves the advantage of a hysteresis modulation's outstanding transient characteristics, and possesses a constant switching frequency. This will help the integration and digital implementation of sliding mode controller for switching DC-DC converter.