<p>In order to elevate the tunnel fire safety, the water mist system has increasingly recognized as a high-efficiency strategy for fire suppression and protection, and it is gradually being widely used in tunnels, especially for the underground and underwater environments. However, the smoke layer is mainly driven by thermal buoyancy, the cooling effect, as well as the resistance caused by the descending droplets can cause the smoke layer to become unstable, which are harmful for persons evacuating buildings and fire rescue. This study numerically investigates the cooling and dragging influences exerted by water mist on the accumulated smoke layers in tunnel fire, and the most typical water mist parameters of flow rate of water and the particle diameter of water are taken into account. The findings show that once the nozzles are activated, the phenomenon of smoke logging emerges and it causes a distinct fluctuation in smoke propagation along the tunnel. An increase in the water flow rate tends to exacerbate the smoke logging, while an increase in the water particle droplet size tends to reduce it. The stability of smoke layer can be characterized by the ratio of the drag force exerted by water mist to the buoyancy force of smoke layer (<i>D</i>/<i>B</i>), and a critical value of 0.14 can be found for the condition where the smoke layer loses its vertical gradient of temperature and descend to the floor level, at which point it appears completely out of stability. Understanding and predicting the stability of smoke layer are vital for refining water mist strategies and enhancing the overall safety measures within tunnel environments.</p>