The　main　structure　of　Guangdong　Jinma　Bridge　is　a　kind　of　special　combined　system　of　T　shape　steel　structure　and　single　tower　cable　stayed　bridge.　The　successful　implementation　of　this　special　structure　has　drawn　more　attention.　In　this　paper,　based　on　the　generalized　potential　variation　principle　of　nonlinear　elasticity　theory　with　large　deflection,　the　incomplete　generalized　potential　energy　functional　with　large　deflection　is　established　on　the　space　coupling　free　vibration　of　combined　structure　by　considering　the　effect　of　axial　compressive　and　shearing　strain　energy　of　stiffening　girder.　By　constraint　variation,　the　differential　equations　of　longitudinal　vibration,　vertical　vibration,　lateral　vibaration,　and　torsional　vibration　of　the　main　girder　and　the　longitudinal　vibration,　lateral　vibration　of　the　main　tower　have　been　derived.　It　can　provide　theoretical　basis　for　the　analysis　of　free　vibration　of　similar　bridges.