In order to further improve the effect of water mist fire extinguishing system, the atomization characteristics and droplet size characteristics of fine water mist under the presence of flash boiling effect were studied by combining numerical simulation and experiment. The results show that the higher the initial enthalpy of water, the higher the degree of flash boiling, the more severe the secondary crushing effect on the surrounding small droplets after ejecting nozzle, which can reduce the distance of fog flow and expand the radial range. When the flash boiling ratio reaches nearly 30%, more evenly distributed droplets can be obtained, and 90% of the droplets reach the ten micron level, which can achieve a better atomization effect and significantly enhance the fire extinguishing effect. It is concluded that the water mist system using flash boiling excitation can achieve preferable atomization effect.