By the analysis of the reasons for the crack defect appeared during the welding of skirt of equipment with large wall thickness supplied by Russia,this paper explains that in the process of welding,the weld stress increases under the dead weight of the equipment and the overlarge constraint degree of itself structure,and that the welding stress cannot have an effective release. As a result,the welds at the relatively weak side cannot give rise to plastic deformation and the crack defect comes into being in the stress concentration region along the upper side of weld. On this basis,it proposes the effective procedure measures to solve the problems about the defect,and provide reference for welding similar structural parts.