The invention provides a casting mold and forming method for a box body of a gearbox and belongs to the technical field of liquid formation of metal. The forming method comprises the following steps that precoated sand is used for core shooting, so that precoated sand blocks are manufactured, then an outer precoated sand mold layer is formed by splicing the precoated sand blocks, and the outer precoated sand mold layer and a sand core are used for forming a casting cavity of the box body of the gearbox; and finally the box body of the gearbox is manufactured through the casting mold in a low-pressure casting way. According to the specific structure of the casting mold, the sand core is located in the center of the casting mold; the casting cavity is formed in the outer layer of the sand core and mainly formed by the sand core and the outer precoated sand mold layer; an outer resin sand mold layer is arranged on the outer layer of the outer precoated sand mold layer, and the outer precoated sand mold layer and a sand box are connected through the outer resin sand mold layer, so that the casting mold is formed. According to the casting mold and forming method for the box body of the gearbox, the cavity which is formed by the precoated sand and used for large complicated castings is used for low-pressure casting for the first time, the precoated sand blocks are subjected to core shooting under pressure, high-temperature sintering formation is conducted, filling is compact, and the situations that sand filling is incompact and sand adhesion defects are caused due to human factors during resin sand molding are avoided.