The invention provides a forming method of a blow molding hollow box body capable of internally provided with an assembly, which forms the hollow box body by molding sheet-shaped blanks in mold semi-molds, respectively tightly attaches the mold semi-molds to closed molding plate semi-plates through the action matching between the mold semi-molds and an expansion mechanism, the molding plate semi-plates and a base which are arranged in a preformed molding plate device and tightly attaches the sheet-shaped blanks positioned between the mold semi-molds to the mold semi-molds to be molded and preformed through blowing and/or inspiration and performed through the internally installed assembly. The invention also provides the preformed molding plate device, wherein the expansion mechanism and the molding plate semi-plates are arranged on the base; the base is arranged on a guide rail support frame; and a power device drives the base to move on the guide rail support frame. The invention reduces the equipment investment and the product cost and enhances the adaptability of the rough sheet-shaped blanks and the finished product ratio of products; in addition, the internally installed assembly decreases openings positioned on a container wall to the minimum, thereby realizing the low infiltration capacity of the formed hollow box body.