Abstract: In this paper, the fission of interfacial solitary waves in a two-layer fluid system is investigated. It is found that the density ratio of the upper and lower fluids has no effect on the fission conditions of forming two solitary waves. It means that the fission may occur provided that solitary wave travels from deeper fluid to shallower fluid. But the fission conditions of forming more than two solitary waves are related to the density ratio and upstream depth ratio of the upper and lower fluids.