Abstract: High-pressure water mist system is a new type of fire-fighting unit using water hydraulic technology. The advantages of water mist include environmentally friendly， fast and effective fire suppression， minimum quantity of water requirements， low water losses. It has been in fact widely expected the most potential substitute of Halon fire-fighting system. The inner flow state of the vital execute component—direct jet atomizer is analyzed by using axial symmetrical boundary momentum integral method. The atomizer shape dimension’s influence on two inner flow parameters， boundary layer thickness δ， accelerate factor K is assessed. Besides， the outer atomization characteristics related to fire-fighting performance， for instance， outlet velocity， Sauter Mean Diameter (SMD) is analyzed. Simulation results are experimentally verified. The results show that the system working pressure is the most important factor to these outer atomization characteristics and 8~9 Mpa is the appropriate area of stable atomization pressure for this prototype of direct jet atomizer. The atomizer shape dimensions include contract angle a， length of atomizer neck l/d has influence on inner flow parameters and outer atomizer characteristics， but not distinct. The optimum atomizer inner structure is selected.

Key words: Atomization characteristics, Fire-fighting Jet atomizer, Water mist