To solve the problems of stability and load uncertainty of Buck DC-DC converters with constant power load (CPL)，a model predictive control（MPC） strategy with high-order sliding mode observers is proposed. Firstly，the objective function of the Buck converter is constructed according to MPC theory，and the rolling optimization tracking equation is established to solve the optimal control rate. Secondly，a high-order sliding mode observer is constructed to improve the voltage control accuracy and eliminate chattering and the relative order problem. Finally，the stability analysis of small disturbance is carried out to establish the equivalent model of source and load side. When CPL and resistive load are switched frequently，the simulation results show that the MPC control based on high-order sliding mode observer has good dynamics and robustness，compared with the traditional double closed-loop PI adjustment and MPC control. In addition，it can control and track the bus voltage when the load changes.