A modeling method was proposed based on model reference adaptive system (MRAS). According to Lyapunov stability theory, a MRAS was established, with a low-order model which has excellent dynamic characteristics, serve as reference model, and the servo motor system as controlled plant. The low-order model of the servo motor system can be conducted when the adjustable system and reference model achieved an exact match through experiment. Using the proposed method, the second-order model of the servo motor system was derived from the experimental results of case study. The model was proved to have a high accuracy through simulation and experiment. The difference between the two models output within ±0.00063 rad when 50000 pulse signals were input continuously within 20 s. 