Free vibration of summation resonance of suspended-cable-stayed beam is investigated in the article. A 3-DOF model of the coupled structure is built, with the main cable and sling (vertical cable) considered to be geometrically nonlinear, and the beam is taken as linear Euler beam. Hamilton's principle is used to derive the dynamic equilibrium equations of the coupled structure. Then, the dynamic equilibrium equations are solved by means of multiple scales method, the second order approximation solutions of single-modal motion of the coupled structure are obtained. Numerical examples are presented to discuss time history of free vibration of the summation resonance, with and without damping. Additionally, fourth-order Runge-Kutta method is directly used for the dynamic equilibrium equations to complement and verify the analytical solutions. The results show that the coupled structure performs strongly nonlinear and coupled characteristics, which is useful for engineering design.