In this article, the problem of actuator fault detection for the discrete‐time switched system is considered based on the delta operator approach. The procedure can be carried out effectively in the presence of the unknown input, the outside disturbance, and the actuator fault by using the observer‐based method and the mode‐dependent average dwell‐time (MDADT) technique. In order to detect the actuator fault, an unknown input observer (UIO) is employed as the residual generator. Then, sufficient conditions are obtained to guarantee that the error system is asymptotically stable with an performance by means of the MDADT method and model‐dependent Lyapunov function technique. Furthermore, a specific design algorithm of the UIO is proposed. Finally, a circuit system is borrowed to demonstrate the effectiveness of the obtained techniques.