This paper considers an approach about active fault diagnosis with a new framework for closed loop system. Firstly, a new framework of active fault diagnosis is proposed with a reference model for closed loop system. Secondly, different from the traditional design methods, peak of the amplitude of the responses from the auxiliary signal on the output, active duration of the auxiliary signal on the system and the effects of the auxiliary signal on the control signal are proposed to formulate the auxiliary signal. A constrained multi objective optimization problem is formulated to contain all the criteria. At last, a practical example about DC motor control system DR300 is illustrated to validate the effectiveness of the proposed method.