When the bottom plate and the Ⅱ-beam are assembled in the crane major beam production line, the manual rectification process will produce scars on the girder web that need additional polishing operation. In order to improve the condition and cooperate with the robot to deal with the tack weld, the method of magnetic adsorption was used to realize the automatic rectification for the girder web deformation. Firstly, the deformation of the girder web was measured, and the deformation curves were fitted. All deformations were concave, and the max value was 24 mm. Secondly, the effect of the layout of the magnet was analyzed by the finite element method. Under the same magnetic adsorption stress and the same magnet layout, the larger the span between the big rib plates were, the better the rectification effect became. And under the same span between the big rib plates, the small initial deformation was beneficial to rectify. To all grid space, the nearer the distance from the lower edge of the girder web was, the better the rectification effect became. The horizontal layout was beneficial to rectify as compared with the vertical layout to the same magnet (the ratio of the length and the width was greater than 1). Finally, the physical prototype of automatic rectification effector was developed, which consisted adsorption unit, push-pull unit, structure unit, sensing unit and control unit, and the field testing was carried out. And the results of the testing were satisfied with the accuracy requirement(±2 mm) of the rectification process. Moreover, the results of the testing also verified the conclusion of the finite element analysis. As a conclusion, the development of the automatic rectification effector realizes the identification of the deformation of the girder web and the temporary establishment with the girder web, the force perception in the rectification process and the control to the rectification motion. The device realizes the automatic rectification of the web deformation before the robot positioning welding, which is of great significance to optimization of welding technology of the previous process.