The　nonlinear　dynamic　responses　of　a　string-beam　coupled　system　subjected　to　harmonic　external　and　parametric　excitations　are　studied　in　this　work　in　the　case　of　1:2　internal　resonance　between　the　modes　of　the　beam　and　string.　First,　the　nonlinear　governing　equations　of　motion　for　the　string-beam　coupled　system　are　established.　Then,　the　Galerkin＇s　method　is　used　to　simplify　the　nonlinear　governing　equations　to　a　set　of　ordinary　differential　equations　with　four-degrees-of-freedom.　Utilizing　the　method　of　multiple　scales,　the　eight-dimensional　averaged　equation　is　obtained.　The　case　of　1:　2　internal　resonance　between　the　modes　of　the　beam　and　string　principal　parametric　resonance-1/2　subharmonic　resonance　for　the　beam　and　primary　resonance　for　the　string　-　is　considered.　Finally,　nonlinear　dynamic　characteristics　of　the　string-beam　coupled　system　are　studied　through　a　numerical　method　based　on　the　averaged　equation.　The　phase　portrait,　Poincare　map　and　power　spectrum　are　plotted　to　demonstrate　that　the　periodic　and　chaotic　motions　exist　in　the　string-beam　coupled　system　under　certain　conditions.