Abstract: Through measuring type-A non-metallic inclusion of sample taken from different depths and location in the steel rail,we found out that that the result is contingent.In this paper taking U75V steel rail as researching object,the separation of MnS in rail steel had been predicted by Factsage software and thermodynamic calculation,,and verified by combining the solidification characteristic of molten steel in the casting process with results of produce statistics and actual testing.It is found out that MnS separates from the nearly full solidified liquid steel,and mainly caused by segregation of S and Mn while molten steel solidifying.Based on the control of decreasing the content of [O]%、[N]%,decreasing the content of [S]% and try to make modification for MnS with Ca or Ti and improve the quality of billets,can help to prevent MnS segregation and deformation while subjecting to rolling.