The stability and nonlinear principal resonance of rectangular orthotropic thin plate excited by a harmonic force were studied in this paper, under the condition of four sides simply supported. Based on the vibration differential equation of orthotropic laminated plates, the nondimensional Duffing nonlinear forced vibration equation was deduced by using Galerkin method. The amplitude frequency response equation of system steady motion under principal resonance was obtained by means of averaging method. Based on Lyapunov stable theory, the critical conditions of steadystate solutions’ stability were got. By some examples, the amplitudefrequency curves, amplitudeexcitation amplitude curves and phase trajectories in moving phase plane were derived under different situations. The stability of solution and the influence of different parameters on nonlinear resonance properties of system were analyzed.