Casting method of back plate casting

The invention relates to a casting method of a back plate type casting. The back plate type casting comprises a casting upper part, a casting lower part and a rib plate for connecting the casting upper part and the casting lower part, the sand mold for casting comprises a first sand mold, a second sand mold and a third sand mold, the first sand mold is used for forming the upper part of a casting, the third sand mold is used for forming the lower part of the casting, and the second sand mold is used for forming a rib plate and bringing out an inner cavity structure of the casting; a plurality of cavity structures are arranged in the second sand mold, and the cavity structures are communicated through air channels; an air inlet hole is formed in the head end of the air passage, and compressed air is introduced into the air inlet hole; an exhaust hole is formed in the tail end of the air passage and is communicated with the outside; and a dead head is arranged on the top surface of the upper part of the casting