In　this　paper,　the　nonlinear　vibration　of　a　thin-plate　workpiece　during　milling　process　is　investigated.　The　thin-plate　workpiece　is　modeling　as　a　cantilevered　thin　plate.　The　equations　of　motion　for　the　thin-plate　workpiece　are　derived　based　on　the　Kirchhoff-plate　theory　and　the　von　Karman　straindisplacement　relations　by　using　the　Hamilton＇s　principle.　By　applying　the　Galerkin＇s　approach,　the　resulting　equations　are　reduced　to　a　two-degree-of-freedom　nonlinear　system　with　external　excitations.　Considering　the　case　of　1:1　internal　resonance,　the　method　of　Asymptotic　Perturbation　method　is　utilized　to　obtain　the　averaged　equations　of　the　cantilevered　thin-plate　workpiece.　Numerical　method　is　used　to　study　nonlinear　dynamics　of　the　cantilevered　thin　plate　and　get　the　two-dimensional　phase　portraits,　waveforms　phase,　threedimensional　phase　and　frequency　spectrum　phase.　The　result　shows　that　the　cantilevered　thin-plate　workpiece　exhibits　the　complex　dynamic　behavior　with　the　increase　of　the　amplitude　of　the　forcing　excitation.