Addressing the issues of slow reaching speed in traditional sliding mode control, which leads to slow response and low precision when applied in synchronous Buck converters. A method of nonlinear anti-saturation sliding mode active disturbance rejection control is proposed. First, the traditional sliding mode surface function and conventional reaching law were modified by introducing nonlinear and saturation functions to construct an optimal control law. Next, the fal function in the extended state observer and nonlinear error feedback control law was replaced by the optimal control law, completing the optimization of the nonlinear anti-saturation sliding mode active disturbance rejection control. This approach enhances dynamic response performance and improves disturbance rejection capabilities. Finally, a corresponding model was built on the MATLAB/Simulink simulation platform. The results show that the settling time is 138 µs, and the recovery time after a sudden load is 200 µs. Experimental validation also confirms that the proposed method exhibits faster adjustment time and stronger disturbance rejection capabilities when the speed command changes under sudden load conditions.