The types of load model in power system are various, thus the combinations of power system aggregate load model can not be completely listed. In view of the above problems, through the establishment of load model library, a automatic selection method of the optimal load model is put forward in this paper, which regards the minimum fitting error that represents the proximity of fitting response and practical response as objective function, and put the 0-1 logic variables that represents the investment situation of load model as decision variable. The effectiveness and feasibility of the proposed method is verified by the automatic selection of the actual power grid dynamic equivalent model, and the selected model combination can describe the load behavior well.