The assumed mode method is widely used in the dynamic modeling of single flexible structures such as beams, bars, cables and plates. However, its application is limited when dealing with the vibration problems of combined structures because it cannot reflect the coupling between components. The approximate dynamic model of the combined structure can be established by using the assuming modes, and then the natural frequencies and corresponding eigenvectors of the system can be obtained by using the approximate model. Based on this approach, the global modes of the system can be obtained effectively. In this paper, a simply supported beam with multiple elastic supports in the middle span is taken as an example, and the global modes of the system are extracted by assuming mode weighting, so as to establish the dynamic model of the system. The results of the intrinsic characteristics analysis of the system show that the global mode of the system can be easily obtained by assumed model weighting. The results of dynamic response analysis of the system show that the nonlinear dynamic model based on the proposed global mode can effectively reflect the nonlinear dynamic characteristics of the system.