Abstract: The PWM converter of an active power filter has a time-delay link, thus making it difficult to be controlled optimally. Analyzes the PID control principle and the reason why the PWM converter is unable to implement the optimal control, and proposes the way and conditions to realize optimal control with a mathematical model developed correspondingly. A general expression of the state observers to a class of time-delay system is given as well as the ITAE optimal control law. The method combining the ITAE optimal control law with PID controller to realize the PWM converter's optimization control is therefore proposed according to the PWM converter's characteristics. The MATLAB simulation results indicated that the controller can trace signal quickly after ITAE optimization and has better dynamic performance than other optimal methods.