In the dynamic analysis of the assembly structure,there are three methods to simplify the connection between different components: node coupling,rigid / flexible connections and MPC algorithm. Through comparing the accuracy of the three methods in solid model connections and comes to the conclusion that the simplify accuracy of node coupling and MPC algorithm is higher than the rigid / flexible connection,while MPC algorithm is more convenient and practical. The MPC algorithm is applied to the modal analysis of a gearbox assembly,and a good result is got.