Ship engineroom fire is one of the most common and serious disasters in shipwrecks. It has become a hotspot of research to use water mist system to extinguish engineroom fire efficiently in the ship fire fighting field. In this paper, using Fluent to simulate water mist suppressing engineroom fire on the conditions of different fire scales, droplet diameters, spray cone angles and spray pressures. Research indicates that: large scale fires are more easily suppressed than small scale fires by water mist in the enclosure engineroom; the droplets of water mist should be neither too big nor too small, or else it would go against fire fighting. It is pointed out that the droplets should be in the range of 200 μm to 400 μm; the smaller cone angle nozzle can guarantee a high flame cooling effect; the increase of spray pressure can enhance the flame cooling effect, but probably of only marginal help in reducing the oxygen content.