Reviewing the direct current control of four-switch three-phase brushless DC motor (BLDCM), to reduce the system cost further and theoretically explore the system's operating mechanism in more depth, the paper put forward a single-current-sensor sliding mode driving strategy. The uniform mathematic equations of four-switch three-phase BLDCM were represented and six kinds of operating modes (model~model6) were divided up into two groups with model, mode4 classified as the first one and mode2, mode3, mode4, mode5 as the second one. With model and mode2 respectively taken for the examples of the first group and the second group, the fundamental description equations and the standard state space description in every mode were derived and the sliding mode plane, the convergence conditions, and the submode division in each mode were also discussed. In the first type of operating modes a sliding mode control with hierarchical switching scheme in fixed order is adopted, where regulating phase c current then phase a or phase b current successively may direct the system state trajectory toward the two-dimension stable sliding mode plane. Whereas in the second type of operating modes, simply sensing and regulating phase c current can steer the system state trajectory to the one-dimension sliding mode plane. Moreover a phase plane portrait method and a numeric method based on Taylor series expansion to determine the system's convergence domain of the sliding mode plane are also introduced. To verify the feasibility of the plan, a complete simulation system in Matlab6.5/Simulink was built. The whole theoretical analysis and the simulation results verified the validity and reliability of the strategy.