Accurate identification of Thevenin equivalent parameters is the key to realize the power system static stability on-line assessment. It is very difficult to guarantee the assumption that the equivalent parameters are constant between the two moments when the identification method of the two-time power flow is used. Firstly, the existing problems of equivalent parameter identification methods are analyzed, and an improved equivalent parameter identification method based on dynamic equivalent impedance is proposed. The equivalent parameters are calculated on-line quickly from the current operating points and the predicted points. The improved method solves the problem that the original methods are invalid when the load power is constant and the multi-node load power is changed at the same time. On this basis, the static stability margin is evaluated by using the impedance modulus index. Finally, the effectiveness of the proposed method is verified by simulation. This work is supported by National Natural Science Foundation of China (No. 51207143) and Science and Technology Project of State Grid Corporation of China (No. XT71-15-056).