The invention discloses a method for welding a large special-shaped multicavity steel structure, and belongs to the field of steel structure construction. The method includes the steps that the size of the large special-shaped multicavity steel structure is obtained, and the large special-shaped multicavity steel structure is divided into a plurality of sections of splicing units according to the size; a welding sequence is determined according to the structures of all the segmented splicing units; all the splicing units are welded in accordance with the determined welding sequence; welding treatment in corresponding modes is conducted according to the structure of a welding seam formed by every two sections of splicing units in a butt joint mode in the welding process. By the adoption of the method, the reasonable welding length is determined through segmentation, the condition of large welding deformation and the condition of welding stress concentration due to excessively concentrated welding seams are effectively avoided, and the welding work amount inside cavities is reduced to the maximum degree; the reasonable welding seam mode can be helped to be determined through segmentation, damage of operation such as welding and back gouging inside the cavities of the steel structure to operating personnel is lowered, and construction is made to be safer; welding deformation and residual stress are reduced, correcting time after welding is saved, and the welding seam repair rate is lowered.