The equivalent approach combined with FEM was adopted to establish an equivalent dynamic model for a RRR Ⅱ grade bar-group. The key in building the dynamic model is to set up a second order tensor and a third order tenser, as well as to assemble an equivalent mass matrix. A RRR-RRR 6-bar mechanism was taken as example, the procedures of modeling, solving and analyzing processes were demonstrated and the response curves of its dynamics were obtained. The dynamic analysis proved that the method had a high efficiency compared with the conventional ones.