The invention discloses an assembling and construction method for no-section connection and long-span three-main joist beveled-edge joist steel girder cantilevers. According to construction sequences, the method comprises a single-point contraposition rotation method for space three-main joist beveled-edge joist assembling, an axel deviation method for long-span steel girder cantilever assembling, a method for adjusting elevation of space three-main joist beveled-edge joist assembling and a construction method for long-span steel girder suspension assembling, high-strength bolt screwing and steel bridge panel welding. The invention has simple steps and easy construction, and is very suitable for assembling construction and no-section connection of steel joists with a space three-main joist beveled-edge joist structure, which is long-span cantilever assembling and construction for steel girders; single-point contraposition reduces contraposition difficulty, and rotation and accurate contraposition after single-point contraposition are convenient and fast; and the axel deviation method for long-span steel girder cantilever assembling can overcome the technical problem that the axel of steel girder is greatly influenced by construction and not easy to control and adjust during long-span construction. The invention has the advantages of complete measures, practicality and convenience, safety construction, guarantees quality and the like.