Aimed at the problem that maneuvering targets intercepted by interceptor at out of atmosphere are uncertain at endgame guidance, this paper presents a design method based on threedimensional endgame guidance law. The paper establishes a relative motion equation with respect to the line of sight between target and interceptor by adopting a robust guidance law with sliding mode control theory, and designs a guidance law to make the line of sight angular rates in elevation and azimuth engagement planes tend towards zero respectively. The paper integrates the supertwisting sliding mode disturbance observers (SMDO) into the guidance law to compensate uncertainties such as target maneuvers, thus reducing the required switching gains in the guidance law significantly. The simulation results indicate that the proposed sliding mode guidance law is high in guidance accuracy and is good in robustness to maneuvering target.