The 3-D finite element model of an H-beam in straightening process was established by using the processing parameters of a nine-roller cantilever straightener with variable roller spacing. By analyzing the regularity of stress distribution in the H-beam's cross-section during straightening, the character of stress variation was observed which is similar to the stress-wave that spreads from outer-flank to inside-flange, as well as from the two-tips of flange to the middle of web. The simulation result indicates that, the straightening effect is decided by the longitudinal normal stress and the longitudinal shearing stress also affect the distribution of crosssection equivalent stress, while the R comer of web and the area of web contacted with the fillets of straightening rollers suffer from high stress during rear straightening process, and residual stress can also been found after straightening finished.