The invention relates to a long-span box girder cantilever cast-in-place assembled prestressed composite support system, which includes caps, steel tubular columns, pier columns, multi-section steel main girders, section steel distribution beams, unit pre-assembled quick-connecting supports, box girder side formwork, box girder, steel plate steel tubular column scissor braced steel tubular columnpre-embedded positioning supports, long-span box girder cantilever casting composite support system, long-span box girder cantilever casting section reaction preloading system and long-span box girdercantilever casting section formalization support system. The lower part of the steel tube column is connected with the bearing cap through a steel tube column embedded positioning support; a steel tube column shear brace is arranged between each steel tube column-; The upper part of the steel pipe column is provided with a multi-section steel main beam, a section steel distribution beam and a platform steel plate; The upper part of the platform steel plate is provided with a unit-type pre-assembled quick-connecting bracket, and the upper part of the unit-type pre-assembled quick-connecting bracket is provided with a box girder side template. The invention has the advantages of adopting a limit embedded frame plate for guiding installation, effectively improving the installation speed of the steel pipe column, improving the installation precision of the steel pipe column, and convenient construction.