In order to obtain a low-order mathematical model of Delta mechanism servo motor system, a model reference adaptive system (MRAS) modeling method is proposed based on Lyapunov stability theory. In this method , single motion chain in mechanism is treated as whole and served as controlled plant while a low-order model with excellent dynamic characteristics is served as reference model. The low-order model of the Delta mechanism servo motor system can be achieved when the adjustable system is matched accurately with reference model through experiment. To examine the approximation between low-order mathematical model from experiment and the realistic servo motor model, the two models were input with 4000 pulse signals within 16 second .The result show the difference between two outputs was within ±0.000126 radians, which proved the achieved model had a high accuracy.