Abstract: Aiming at a new semi-flexible suspension structure, a simplified calculation method is presented. Based on its characteristics, the modal synthesis method of substructures is applied to the dynamic analysis by considering suspended flexible layer as the link substructure, and the simplified equation of dynamic motion of a semi-flexible suspension structure is given. Under the influence of the lateral stiffness ratio of the flexible layer to suspended floors and the number of modes synthesized, the precision of the simplified technique is analyzed and the time history analysis is applied by this technique. The results show that the degree of freedom participated in structural analysis is decreased by this right modal synthesis method, and the effect of precision is influenced remarkably by the number of modes synthesized. This technique is convenient and effective and can meet the practical demand.