Aiming at the shortcomings of the traditional power system’s Thevenin equivalent parameter calculation method, this paper puts forward a tracing algorithm for the Thevenin equivalent parameters based on power series expansion. The algorithm assumes that the amplitude of the system’s Thevenin equivalent parameters is constant during the adjacent sampling time, and uses the power series expansion to track the Thevenin equivalent parameters. The algorithm not only effectively avoids the parameter drift problem, but also reflects internal load disturbance of the equivalent system to a large extent. At the same time, the algorithm can instantly restore the accurate tracking of the Thevenin equivalent parameter after the system three-phase short-circuit fault, which is available for the transient process after the system short-circuit fault is removed. Finally, the robustness and accuracy of the proposed algorithm are verified by simulations in a simple double bus example system and an IEEE39 node example system. This work is supported by National Natural Science Foundation of China (No. 51507100), Science and Technology Commission of Shanghai Municipality (No. 17020500800, No. 13DZ2251900 and No. 10DZ2273400) and "Shuguang Program" of Shanghai Education Development Foundation and Shanghai Municipal Education Commission (No.15SG50).