Abstract: This paper proposed a novel on-line parameter identification method of Thevenin equivalent circuit that is applicable to the situation when the persistent disturbancesoccur at the system side. Based on the theoretical derivation that the voltage and current variation at a node was taken as the superposition of the variation at the system side and the variation at the load side, a new mathematical model that takes into account the variable at the system side was built. Independence of variables at the system side and the load side was used to solve the model. Compared with the traditional least square method, the new method doesn’t assume that the equivalent source voltage is constant, so it is more applicable to the situation when persistent disturbances occur at the system side. Simulation and filed tests show that the proposed approach performs better than the traditional least square method when the persistent small disturbances occur at the system side. With the persistent disturbances becoming larger, the traditional least square method is out of work gradually, while the proposed approach can identify the parameters of the Thevenin equivalent circuit as well.