Abstract: The parametric and subharmonic resonances of stay cables excited by the vibration of deck are investigated.The mechanical model of cable-stayed beam is firstly established.The nonlinear dynamical equation of cable-stayed beam is then obtained,in which the nonlinearity is caused by the initial sag.The method of multiple scales is applied to the differential governing equations to analyze the parametric and subharmonic resonances of stay cables.The stability of the steady solutions is also investigated.Finally,numerical simulation is used to investigate the parametric and subharmonic resonances of stay cables.The time histories of stay cables under different dampings and initial conditions are obtained.The results of numerical simulation show that the amplitude of cables is related to the damping but not related to the initial condition.