The invention relates to a steel box plate-girder mixed bridge suitable for mountainous conditions. On the basis of the effective optimization for a structure form of a cross section of a girder and the promotion for a mechanical performance of a steel girder, the structure dead load is reduced, the economical efficiency of the structure is increased, the transportation and construction difficulties of the steel girder are reduced and the guaranteeing for the construction quality of the steel girder is benefited. The steel box plate-girder mixed bridge comprises the steel girder and a bridge deck slab fixedly arranged on the steel girder, wherein the steel girder comprises two opening rib steel box girders and at least one inverted T-shaped steel plate girder; the opening rib steel box girders are respectively located on one side of the transverse direction of the bridge; the inverted T-shaped steel plate girder is located between the two opening rib steel box girders; the opening rib steel box girders and the inverted T-shaped steel plate girder are integrally connected and form into a mixed steel girder structure; a steel-concrete combined bridge deck slab is formed by a steel bridge deck slab and a cast-in-situ deck concrete slab; the steel bridge deck slab is welded at the top ends of the opening rib steel box girders and the inverted T-shaped steel plate girder; shear connectors are distributed on the steel bridge deck slab; the cast-in-situ deck concrete slab is formed by the cast-in-situ concrete on the steel bridge deck slab.