Abstract: The section deflection of an H-beam is not considered in the traditional straightening theory, so it is difficult to get desired straightening effect. A finite element model of one straightening unit was established with ANSYS, the straightening process was simulated and the section deflection of the H-beam was studied. The results show that the section deflection of the H-beam in straightening process should not be neglected, in which web falling plays a dominating role. The main factors which influence web falling include straightening force, gap between roller outside and beam inner side and roller-outside-corner radius, and the varying regulation of web falling is quadratics of the relevant factors. At last, a regressive formula of web falling was presented. In order to obtain an ideal straightening effect, the actual bending deflection should be the sum of the theoretical bending deflection and the web falling.