Abstract: Urban Traffic Link Tunnel(UTLT) is a new-type underground traffic system and smoke control is important to ensure fire safety in tunnels. A dimensionless equation of back-layering in UTLT tunnel with scaling analysis method was derived. According to usual sizes of UTLT,tunnels of different sizes were set and then fire was simulated with CFD software FDS 5.5.The length of back-layering was obtained at different conditions.At last a model of back-layering in UTLT was obtained. The model agreed well with full scale and medium scale experimental data. The model of confinement velocity was derived on the basis of back-layering model.The research can provide technical reference for UTLT fire protection design.