The flash evaporation process is widely used in industrial production processes such as seawater desalination, ocean thermal energy conversion or waste heat utilization of waste liquids. In this paper, the flowing flash evaporation process in a large-size flash chamber is simulated numerically by a modified Lee model coupled with the volume of fraction model. Considering the pressure-driven phase change in the flash evaporation for water, the modified Lee model is applied, which changes its phase transition temperature according to the static pressure at different locations. The results show that the mass rate of vapor generation increases with the increasing inlet superheat and initial water level height under the fixed apparent retention time, while the flash evaporation efficiency increases with the increasing inlet superheat and decreases with the increasing initial water level height. For different inlet superheat, the different bubble motion patterns caused by the different rates of phase change around the inlet of the flash chamber lead to a significant differences in the flow and the temperature distribution inside the flash chamber, which greatly affects the flash evaporation process by influencing both the degree of breaking of the upper water in the intense flash region and the actual retention time of the water.