Aerial firefighting is a common measure to extinguish forest fires and is expected to be helpful in managing urban disasters. Numerical analysis is an effective approach in the development of a firefighting aircraft because numerous tests with different conditions including aircraft speed, water amount, and fluid properties can be conducted at low cost. A combination of the Volume of Fluid method and the Discrete Droplet Model is widely used, but the method requires theoretical or empirical models that affect the calculation result. In this paper, as an alternative approach, the grid–particle coupling method is applied to numerical analyses of water spray from a firefighting aircraft. In the method, gas and liquid phases are calculated by grid and particle methods respectively, and they are coupled using the Aerodynamic Force Model developed by the authors. Results show that water spray behavior such as column breakup and surface breakup, and features of the waterfall distribution on the wind tunnel floor, can be qualitatively captured. Introducing a sub-grid scale aerodynamic model was effective in enhancing atomization and obtaining a successful result.