The hazards of the spread of fire smoke in the tunnel have been well recognized. This paper study the effects of smoke confinement by using water spray in the tunnel at natural ventilation. A 1/20 reduced scale tunnel was built with a dimension of 6000 mm length × 640 mm width × 360 mm height. Smoke temperatures, densities at different location were measured. Individual smoke clusters/particles were captured by brass meshes and imagined by Scanning Electron Microscope. The results indicate that, water spray can not only cool the hot smoke down by evaporation of spray droplet but also removal the smoke particles by the absorption of spray droplets. The temperature records behind the back of water spray decreased below 50℃ after the water spray is activated. The smoke removal efficiency of water spray increased with nozzle pressure. The removal efficiency could reach to 99.7% when the nozzle pressure was set at a relative high pressure. Additionally, the mechanism of absorption of smoke particles by water droplet was discussed. A simplified model was presented to predict the absorptions. The critical condition was determined that depend on the surface tensions of particle/air, particle/water and air/water.