The nonlinear dynamics of a fouredge simply supported rectangular thin plate under the combination of the parametrical and external excitations were investigated. Based on the von Karman theory, the formulas of motion for the fouredge simply supported rectangular thin plate under the combination of the parametrical and external excitations were derived. The partial differential equations were discretized to the ordinary differential equations with threedegreeoffreedom using the Galerkin approach. Considering the resonant cases of 1:2:4 internal resonance and principal parametric resonance1/2 subharmonic resonance, the method of multiple scales was utilized to obtain the sixdimensional averaged equations. Furthermore, numerical method was carried out to investigate the periodic and chaotic motions of the thin plate. The results show that the chaotic responses of the thin plate are sensitive to the external excitation.