The　design　of　reasonable　flat　ratios　for　the　urban　large-span　bifurcation　tunnel　is　very　important　for　cost　saving　and　structure　security.　As　the　span　of　the　bifurcation　tunnel　is　too　large,　a　new　structure　type　is　adopted　in　this　paper.　In　this　structure,　a　vertical　steel　support　connected　with　the　top　and　bottom　longitudinal　beams　is　set　to　ensure　the　safety　and　stability　of　structure.　According　to　this　new　structure　type,　5　different　flat　ratios　are　selected.　Based　on　load-structure　model,　the　MIDAS-GTS　NX　software　is　used　for　numerical　analysis　to　obtain　stress　characteristics　and　safety　factors,　and　to　optimize　the　flat　ratio.　The　results　show　that　the　maximum　positive　and　negative　bending　moment　of　the　structure　appeared　in　the　left　spandrel　and　the　joint　of　the　vault　and　steel　support　in　each　condition.　With　the　increase　of　flat　ratio,　structural　internal　force　decreases　correspondingly.　The　safety　coefficient　is　large　of　the　structure　with　low　discreteness　and　small　quantity　of　reinforcement.　However,　the　increased　excavation　area　results　in　construction　diseconomy.　Therefore,　the　final　relatively　optimal　selection　of　the　flat　ratio　is　0.57,　in　order　that　the　security　and　economy　can　be　guaranteed.　©　2015　ejge.