An in-plane nonlinear vibration equation of stayed-cable motion under axial displacement excitation was introduced. Sag of cable, inclination angle, large displacement, excitation amplitude, damping coefficient and etc influencing factors were introduced into the equation. The equation was solved by Runge-Kutta Method. An Experimental model of stayed-cable is established and tested in lab. Parametric oscillation of the cable was observed first in. The results of numerical calculation and experimental data show that parametric oscillations appear in a special range of frequency ratio, amplitude-frequency curve of cable vibration is nonlinear, damp of cable is able to increase the threshold amplitude of excitation for parametric oscillations.