For the mower motor torque ripple control process exists, and poor dynamic performance, the design of the vector system speed brushless DC motor mower. Instead of using the PI control sliding mode controller for adjusting the speed of the motor, and a position estimation system were designed based on the high frequency square wave oscillator pulse injection SMO proposed a revolution in the motor mechanical angle analysis Within the two errors, the strategy of switching the position estimation method realizes the position estimation of the motor at full speed. Simulation in MATLAB, and take advantage of STM32F302 chip built hardware test platform. The results show that the two rotor position estimation methods can be switched smoothly, which improves the accuracy of the rotor position estimation. The use of the sliding mode controller reduces the speed adjustment time and improves the robustness of the system.