In　this　paper,　the　bifurcations　of　subharmonic　orbits　are　investigated　for　six-dimensional　non-autonomous　nonlinear　systems　using　the　improved　subharmonic　Melnikov　method.　The　unperturbed　system　is　composed　of　three　independent　planar　Hamiltonian　systems　such　that　the　unperturbed　system　has　a　family　of　periodic　orbits.　The　key　problem　at　hand　is　the　determination　of　the　sufficient　conditions　on　some　of　the　periodic　orbits　for　the　unperturbed　system　to　generate　the　subharmonic　orbits　after　the　periodic　perturbations.　Using　the　periodic　transformations　and　the　Poincar,　map,　an　improved　subharmonic　Melnikov　method　is　presented.　Two　theorems　are　obtained　and　can　be　used　to　analyze　the　subharmonic　dynamic　responses　of　six-dimensional　non-autonomous　nonlinear　systems.　The　subharmonic　Melnikov　method　is　directly　utilized　to　investigate　the　subharmonic　orbits　of　the　six-dimensional　non-autonomous　nonlinear　system　for　a　laminated　composite　piezoelectric　rectangular　plate.　Using　the　subharmonic　Melnikov　method,　the　bifurcation　function　of　the　subharmonic　orbit　is　obtained.　Numerical　simulations　are　used　to　verify　the　analytical　predictions.　The　results　of　the　numerical　simulation　also　indicate　the　existence　of　the　subharmonic　orbits　for　the　laminated　composite　piezoelectric　rectangular　plate.