Based on the configuration and mechanical characteristics of the cable-stayed bridge, a dynamic multi-beam dynamic model with discrete springs is proposed for evaluating the in-plane vertical bending stiffness of the multi-tower cable-stayed bridge with hybrid system. First, based on the relevant achievements of our group, the treatment methods of cable-stayed bridges with different systems by transfer matrix method are introduced. Then, by using the continuity and boundary conditions, the basic theory for the evaluation on the vertical bending stiffness of the multi-tower cable-stayed bridge with hybrid system is established. Finally, taking the Chishi bridge as an example, the corresponding finite element model is established and the frequencies calculated by the method in this paper are compared with those by the finite element method and field measurement. The results show that the method in this paper is feasible and can be used for the evaluation of the vertical bending stiffness in the multi-tower cable-stayed bridge with hybrid system.