The sensitivity coefficient of damping torque analysis(DTA)reflects the influence on system modalities of the change in transfer function. However

the coefficient is the value in the open-loop state. The value in the closed-loop state is more meaningful. As a result

this paper proposes the closed-loop DTA and its calculation method. Based on this

a new sensitivity index is proposed. Its application scenarios are also put forward including the closed-loop parameter design of power system stabilizer(PSS)and the online optimization of PSS when the operating method changes. At last

the correctness and feasibility of the closed-loop DTA and its applications are demonstrated by practical examples. This work is supported by National Natural Science Foundation of China(No. 51277029).