Abstract: A thin layer vapor film due to the rapid evaporation of liquid around the particles in the coarse mixing stage surrounds high temperature particles.The flow in the boundary layer around the high temperature particles is greatly different from the flow without evaporation,and then the general drag model of particles in room temperature cannot be adopted in this situation.In this paper,considering the forces distribution and surface evaporation effect,an evaporation drag model for one high temperature particle moving inside coolant is proposed based on the analysis of acting forces on the particle.It is theoretically indicated that the thickness of the vapor film has great effects on the drag force.In the beginning of vapor film growing,the evaporation drag is always much larger than that in general conditions except the relative velocity is very high.The calculation results prove that unless the initial film thickness or the relative velocity is very large,the inner evaporation drag should be taken into account at high temperature conditions.