Abstract: Based on the boundary layer stripping mechanism, the hydrodynamic fragmentation process of fuel and coolant interaction（FCI） has been studied. With theoretical modeling and experimental correlation, semi-empirical equations which can predict the mass fragmentation rate and average size of fragments have been proposed. It can be intuitively referred from the equations that the relations between the hydrodynamic fragmentation process and the relative Weber numbers.The rationality of the equations is verified by comparing with typical experimental results. All the theoretical work is done for application in program development of FCI in the future.