To propose a precise and convenient solution method for in-plane vertical modes of cable-stayed bridges， a new dynamic model comprised of lumped mass beam segments was established in this paper， which was used to simulate the in-plane vertical natural vibration behavior of cable-stayed bridges. In the model， the cables were reduced to vertical elastic supports and external forces on the girder in the horizontal direction. Additionally， the shear forces on both sides of the micro beam-segment were introduced to simulate the actions of the beam’s bending stiffness and the coupled interaction between the cables. Based on the moment equilibrium between the beam segments and the finite difference method， the solutions of the in-plane vertical modal frequencies and shapes of cable-stayed bridges of different systems can be obtained. A computational solution program was also developed. The theoretical solutions were compared to field-test results from an actual bridge， confirming that the model and method proposed in this paper can accurately calculate the modal properties of in-plane vertical modes of cable-stayed bridges. The results through parametric analysis showed that the effect of the beam’s axial force was more significant on the low-order modal. The phenomenon of a transition on the low-order mode was observed when the axial force was increased to a high value. Moreover， when the cable is broken， the influence on each mode is related to the corresponding mode participation coefficient of the beam segment anchored with the cable.