spring system. Abstract. In order to increase the efficiency of system dynamic modeling and simulation, the order reduction method of system dynamic equations based on vector bond graph is proposed. Based on system independent energy storage field, resistive field, source field and junction structure, the different unified formula for order reduction of system dynamic equation is derived. Thus, the different order reduction method can be selected by the need of practical problem. The proposed method is organized and very suitable for computer aide dynamic analysis. Based on this, the computer aided order reduction of system dynamic equations for a mass spring system is realized, and the validity of this procedure is illustrated.