The　prediction　smoke　back-layering　length　and　critical　velocity　in　the　event　of　a　fire　in　a　utility　tunnel　are　studied.　In　this　paper,　the　method　of　numerical　simulation　is　used　to　study　the　combustion　characteristics　of　the　cable　tray　in　a　utility　tunnel.　The　prediction　of　smoke　back-layering　length　is　related　to　factors　such　as　heat　release　rate,　longitudinal　ventilation　velocity,　and　fire　source　location.　The　smoke　back-layering　length　is　determined　by　the　temperature　record.　The　temperature　measurement　is　made　by　thermocouple　tree　in　the　utility　tunnel.　Results　show　that　the　dimensionless　critical　velocity　has　a　certain　influence　on　the　dimensionless　heat　release　rate　in　utility　tunnel.　The　numerical　simulation　results　show　that　there　is　an　exponential　relationship　between　the　dimensionless　smoke　back-layering　length　with　two　fire　locations　(locations　A　and　C)　in　the　utility　tunnel.　This　paper　proposes　a　method　for　predicting　the　smoke　back-layering　length　based　on　the　correlation　of　numerical　simulation　data.　In　two　fire　locations　(locations　A　and　C),　the　numerical　simulation　data　of　the　smoke　back-layering　length　has　a　good　correlation　with　the　results　of　the　Li　model.　The　main　contribution　of　this　paper　is　to　prevent　the　smoke　propagation　in　utility　tunnels　and　provide　technical　guidance.　Therefore,　this　paper　has　certain　guiding　significance　for　the　smoke　propagation　below　the　ceiling　of　the　electric　compartment　in　utility　tunnel.