In the process of the heavy rail roller straightening, the roller leveller only presses the plastic deformation on the web of heavy rail. Because the stiffness of flanges which are in both sides is stronger than the web, the heavy rail always has the phenomenon of web falling. In that case, we can not achieve the desired effect of straightening by the theoretic bending deflection. In this paper, a finite element model of one straightening unit was built by the ANSYS, the straightening process was simulated and the web falling affect with the bending deflection and the roller spacing was analyzed. Some pictures of curve were drew through the relationships of bending deflection and web falling, the relationships of roller spacing and web falling. In order to gain the better straightening effect, the actual bending deflection can be the sum of the theoretical bending deflection and web falling from the curve once the roller spacing was defined.