Abstract: To calculate vertical frequency of three-span continuous system suspension bridge conveniently, the three-span continuous system suspension bridge with double-towers was taken as research object. Frequency formulas for first symmetric and asymmetric vertical vibration modes were deduced by the Rayleigh method considering tower stiffness influence, and the influence coefficient of tower stiffness was proposed. Finally, the proposed formulas were validated by the engineering project, and the application condition was discussed. The results indicate that tower stiffness has a great influence on first symmetric vertical frequency, which should be considered in frequency calculation. The extent of influence can be assessed by the main tower stiffness coefficients formula. The error between values calculated by the proposed formula and the finite element method (FEM) satisfy the requirement of conceptual design. The proposed formulas can be applied to the simple-supported single or three-span continuous suspension bridge.