Abstract: The question of discrete-time variable structure controller design for uncertain systems is researched based on the reaching law approach. An adaptive switching gain which can be automatically tuned according to system uncertainties is designed and the robust stability of the closed-loop system, as well as its quasi-sliding mode motion, is discussed. Theoretic analysis and simulation results show that the proposed method can decrease the chattering and improve the performance of discrete variable structure controller effectively. Also, the limitation for the convention design method, which requires the boundary of system uncertainties be known, is dismissed in the proposed method.