As an important fire-fighting method for ships, the effect of water mist on the deflagration accident of methane mixed with hydrogen in confined spaces such as the engine room and upper deck of the ship is still unclear, and there is a lack of relevant experimental research and explosion suppression mechanism analysis. In view of this, a premixed gas detonation experimental platform was built in this paper, and the mechanism of explosion suppression was analyzed by changing the premixed gas composition, ultrafine water mist particle size, spraying position of water mist and the presence of obstacles. The results show that the explosion suppression effect of water mist is affected by the instability of premixed gas combustion, obstacles and the spraying position of water mist. In the early stage of flame development, 8μm water mist showed the best suppression effect, the peak flame velocity was reduced by 37%, and the peak overpressure was reduced by 36%. 14%. Under the condition of obstacles, changing the spraying position of water mist, the spraying of 45μm water mist near the obstacle showed the best suppression effect, the peak flame speed was reduced by 20%, and the peak explosion overpressure was reduced by 35%. According to this study, in the arrangement of ship fire protection system, the instability of premixed gas combustion, obstacles and the spraying position of water mist should be fully considered, and 8 μm water mist should be used in the early stage of flame development, which has the least impact on the structure of the flame surface, 45μm water mist should be used near obstacles to minimize accident consequences.