In this paper, a mathematical model of multi-stage flash (MSF) chamber was proposed. The volume of fluid (VOF) multiphase model was modified by adding the quality source term, the component source term and the energy source term in the governing equation through the user defined function (UDF) program. The Fluent software was adopted to solve the proposed model. By analyzing the vapor-liquid flow nephogram of velocity vector and mass fraction inside the flash chamber, the relations between the vaporization rate and the structure of the flash chamber were explained. Two approaches for enhancing the flash efficiency were suggested, i.e., increase the spoiler to prevent liquid from flowing directly into the next stage through the bottom outlet and, change the vertical baffle angle to reduce the flow dead zone in the low pressure chamber. Based on this, five improved flash chamber structures were proposed. Simulation results showed that the flash efficiency can be significantly increased by adding the baffle and/or by changing the angle of the vertical baffle. The flash rate of Model-4 and Model-6 are higher than others. In the MSF seawater desalination process, Model-6 is suitable for low concentration stages, while Model-4 is ideal for high concentration stages.