Abstract: Based on the nonlinear vibration equation of a unilateral plate partially immersed in fluid and moved in the axial direction, which is derived from von K醨m醤 thin plate equation with large deflection, the 1:3 internal resonances characteristics of the unilateral plate under an external excitation are studied. The speed and tension of the unilateral plate in the axial direction, fluid-structure interaction and damping are considered. Galerkin method is used to disperse the vibration equation. The nonlinear modal equations are solved by applying numerical and approximate analytical techniques, respectively, and complex frequency-response curves with internal resonance are obtained. The stability of periodical solutions is discussed. At last, the bifurcation phenomenon of the averaged equations with 1:3 internal resonances is studied.