The invention discloses a combined box girder and a construction method thereof. The combined box girder comprises top plates, a left web, a right web and a U-shaped steel member; closed cavities are arranged at the two sides of the U-shaped steel member, the left web and the right web are installed at and fixed to the top faces of the closed cavities respectively, the top plates are fixed to the left web and the right web, and a groove of the U-shaped steel member is filled with concrete. The combined box girder is reasonable in structure, large in rigidity, high in durability and small in depth-span ratio. The invention further discloses the construction method of the box girder, construction adopting the method is safe, high in quality, convenient and fast. The combined box girder is applicable to projects of municipal bridges with jammed traffic, large-span highway overpass bridges and the like.