Network Control Systems Security - This paper is concerned with the observer-based control design for a continuous linear networked control systems under denial of service attacks. In order to save network communication resources, a new flexible event-triggered control strategy is designed on the premise that denial of service attacks are power-limited pulse width modulation interference. Considering this influence of denial of service attacks on event-triggered state, the maximum system performance lost is calculated. The sufficient conditions of system stability are derived by using the Lyapunov functional method. The constructive design of the controller is expressed in terms of linear matrix inequalities. Finally, the theoretical results are verified by a simulation example.