An interval observer that can realize the joint interval estimation of the state and actuator fault is proposed for networked control systems under stealthy attacks. Firstly,the upper and lower bounds of the attacks are obtained according to the characteristics of the stealthy attacks. Secondly, the augmented system equivalent to the original system is obtained by considering the actuator fault as augmented state. Based on the obtained augmented system, the L ∞ performance index is used to suppress the interference and for the impact of the attacks, a robust augmented state interval observer is designed to obtain the close interval estimation information of the state and actuator fault of the attacked system. Finally,the simulations verify the effectiveness and practicability of the proposed method.