As the application of lithium ion battery more and more widely, the safety of the lithium ion battery is more and more attention from society. In order to avoid secondary disasters caused by thermal runaway of lithium-ion batteries for energy storage, installing a water mist device in the battery pack can effectively inhibit the occurrence of thermal runaway in the early stage of thermal runaway, and on the other hand can effectively extinguish the fire caused by thermal runaway. In this paper, a water mist probe was installed directly above a typical battery pack small space, and based on the N-S equation, coupled DPM model, wall film sub model and discrete phase boundary conditions, to simulate the diffusion of water mist in the battery pack film forming process. The study found: In the battery pack space with an internal void size of 1500 mm × 1300 mm × 5 mm, the diffusion film formation process of high-pressure water mist with a spray flow rate of 0.026 kg/s and a pressure of 2.5 Mpa can be roughly divided into four stages. In the late stage of liquid film formation, there will be a phenomenon of middle clustering and diffusion on both sides. Under this condition, the surface water film of the battery can completely cover the space of the battery pack within 150 s.