A numerical analysis was carried out to study the gas-steam ejection system. For the analysis, pressure based solver, realizable - turbulence model, and standard wall function were used. In order to simulate the interaction of hot gas and coolant, which are different materials, VOF (Volume Of Fluid) model and Discrete Phase Model Was used. To simulate the motion of the projectile, a dynamic grid system was used to expand the analysis area in conjunction with the mixture-gas pressure information inside the breech. Pressure-velocity coupling method of PISO (Pressure-Implicit with Splitting of Operators) algorithm is used to prevent calculation of inaccurate pressure gradient due to oscillating pressure value. The gradient of the scalar value for the spatial discretization uses the least squares cell-based method and the discretization for each physical value uses second-order upwind scheme. The time discretization for the transient calculation was performed using the first-order implicit method. In the same hot gas condition, gas-gas analysis assuming a very fast coolant evaporation rate and gas-liquid analysis assuming liquid coolant were respectively performed to predict the temperature, pressure, streamline, coolant distribution and evaporation of the mixture-gas in the canister breech, and the acceleration and velocity of the projectile. As a result, the spatial temperature distribution inside the breech was influenced by the mixture-gas flow and distribution of coolant. The temperature of mixture-gas varied greatly depending on the presence or absence of coolant. but the rate of decrease in pressure tended to be lower than the rate of decrease in temperature. This tendency was more showed when the mass flow rate of the mixture-gas was increased. Therefore, it was confirmed that the key design factor for optimizing the gas-steam ejection system is to minimize the pressure drop and maximize the cooling effect by controling the evaporation of the coolant.