In this study， a refined in-plane dynamic model of cable-stayed bridges was constructed， and the corresponding free vibration characteristics were analyzed. Firstly， the in-plane motion equations of the cable-stayed bridge were derived using the Hamilton variational principle， and the characteristic frequency equation of the linearized model was determined based on boundary conditions. A three-span cable-stayed bridge with double towers was chosen for the numerical calculation， and the correctness of the corresponding solution was verified through the finite element method. Then， the modal characteristics of the cable-stayed bridge were quantitatively reflected by introducing localization factors. Finally， the effects of structural parameters， cable-deck interaction， and structural systems on the natural frequencies and modes were discussed. The results show that the natural modes of the system exhibit the local characteristics when the natural frequencies of the cable-stayed bridge are close to the ones of pure cable. Moreover， the cable-deck interaction may significantly affect the low-order non-local modes. However， the effects on the higher-order frequencies can be ignored.