The　smoke　propagation　in　branched　tunnel　is　more　complicated　because　of　the　special　bifurcation　structure　that　has　not　been　clarified,　despite　the　fact　that　the　scenario　exists　in　practice.　In　this　paper,　the　thermal　smoke　movement　behavior　in　branched　tunnel　fire　were　experimentally　investigated　under　the　combined　effect　of　bifurcation　angle　and　ventilation　velocity.　The　theoretical　analysis　was　conducted　to　develop　the　smoke　backlayering　length　equation　based　on　the　applied　force　of　smoke　propagation　upstream　of　the　fire　source,　and　the　smoke　back-layering　length　under　various　longitudinal　ventilation　velocities　was　quantified.　The　results　indicate　that　the　smoke　back-layering　length　decreases　with　the　increasing　of　longitudinal　ventilation　velocity　for　a　given　heat　release　rate.　The　stronger　heat　release　rate　results　in　the　longer　smoke　backflow　distance　under　same　ventilation　velocity.　The　smoke　back-layering　length　varied　with　bifurcation　angle　due　to　the　bifurcation　structure　weakens　the　inertial　force　of　longitudinal　ventilation,　but　the　affected　degree　varies　with　bifurcation　angle.　The　experimental　smoke　back-layering　length　can　be　well　correlated　using　the　deduced　logarithmic　function.　The　empirical　model　taking　the　bifurcation　angle　into　consideration　is　proposed　to　predict　the　smoke　back-layering　length　in　branched　tunnel　fire.　Finally,　the　correlation　for　critical　ventilation　velocity　of　branched　tunnel　fire　under　varied　bifurcation　angles　was　also　proposed　based　on　the　boundary　condition　of　none　smoke　back-layering.