The invention relates to a fully fabricated steel structure connecting assembly and a mounting method, and belongs to the technical field of building structural components. The fully fabricated steelstructure connecting assembly comprises two square steel pipe columns, outer ring plate assemblies and connecting members. An interlocking broken-line-form concave-convex groove is formed in the jointof the two square steel pipe columns. The outer ring plate assemblies comprise horizontal single plate assemblies and vertical plate assemblies. A plurality of bolt holes are formed in a square steelpipe column wall, inner plates, outer plates, horizontal plates and connecting plates, the square steel pipe column and the outer ring plate assembly are connected by bolts, and the outer ring plateassemblies are connected to each another by bolts and sliding groove devices. The fully fabricated steel structure connecting assembly prevents the occurrence of local stress concentration phenomenon,improves the stress performance of joints, and enhances the rigidity of the joints and components in the horizontal direction, so that the stress distribution in the joint region is relatively uniform, the rigidity is large, the bearing capacity is high, and initial defects caused by welding and possible weld joint tear damage are avoided.