A second-order sliding-mode controller based on improved sub-optimal algorithm for synchronous Buck converter is designed, and the converter can be controlled only by detecting its output voltage without measuring the capacitance current of the converter. By introducing hysteresis to sub-optimal algorithm,the switching frequency of the converter is limited when the output voltage is stable. In the Simulink,the second-order sliding-mode controller is actualized by using the state machine, and the simulation test verifies the start-up performance, the steady-state behavior and the robust against the large load disturbance jump from 10 A to 20 A and the power disturbance jump from 5 V to 10 V. The simulation results show that the proposed second-order sliding-mode controller retains the robustness of sliding mode control and is superiors to first-order sliding-mode control in the aspect of resisting disturbance.