The invention discloses a temporary reinforcement method for transverse partitioning construction of a steel box girder bridge. The temporary reinforcement method particularly includes the following steps that after transverse partitioning of steel box girders, the two sides of each girder block web are each provided with a diagonal bridging at the interval of one longitudinal reinforcement rib, and the girder block webs are arranged in the cutting positions of the steel box girders; and the fulcrums of the diagonal bridgings are arranged at the joints of transverse partitioning plates and top and bottom plates. After the diagonal bridgings are added, the longitudinal flexural rigidity and transverse torsional rigidity of each girder block basically keep consistent, so that the safety in the hoisting process is guaranteed. After the girder blocks are hoisted to a temporary support, the displacements of the girder blocks are basically consistent, and the displacement difference in the longitudinal welding position and the post-welding residual stress are reduced. The top and bottom plates are welded in a staggered mode, the fatigue stress of a welding seam is increased, and the construction safety of the structure is facilitated.