Abstract: The critical buckling stresses of web plates with boundary elastic rotational restraints under single shear or single bending can be obtained by Rayleigh-Ritz method, while in the case of web plates under composite shear and bending, the expression will become too complicated to solve. To solve the boundary rotational restraint web buckling under composite shear and bending, a finite element modal of a web plate with restrained flanges was built to analyze how the bending-shear correlation curve was affected by the flanges restraint coefficient, the ratio of shear-bending , the bending stress gradient λ and aspect ratio . Then a new bending-shear interaction equation of a web plate with restrained flanges was found by the finite analysis, the results had a good agreement with that of a steel-concrete composite girder experiment, the error was less than 2.5%.