The invention relates to a large span steel box girder cable-stayed bridge tower lower beam segment mounting method, belonging to the bridge construction steel box girder construction technology field. The construction method comprises the steps: (1) setting up a masts type crane lifting tower area steel box girder by means of the upper crossbeam of the cable tower and the construction wind; (2) mounting the bridge deck crane in on the tower area steel box girder and lifting the first standard beam segment immediately adjacent to the tower area steel box girder; (3) mounting the bridge deck crane symmetrically on the mounted first standard beam segment, and starting the standard beam segment symmetrical hanging on both the river side and the bank side. The method in the invention achieves the advantages of simple, practical, safe, reliable, economic, convenient less limited by the construction condition, solves the problems in present large span steel box girder cable-stayed bridge construction process.