Abstract: Responding to the needs of online simulation analysis of modern power grid, a novel online electric load modeling method based on interactive multiple model（IMM） algorithm was proposed. Firstly, the framework of online electric load modeling was established according to the basic theory of IMM algorithm. Then the specific steps,e.g., the input interaction, conditional filter, probability updating and fusion estimation of sub-models, were all presented for the online electric load modeling. In the proposed method, the complex characteristics were simulated by some sub-models according to both offline and online information. Therefore, the nonlinear discrete change of electric load is described, resulting to the fast modeling speed and high modeling accuracy. The simulation results demonstrate its effectiveness in the end.