The invention belongs to the field of sand mold cast steel molding materials, and particularly provides a composite casting technology for hardening alkaline phenolic resin sand and water-glass sand by using CO2. The composite casting technology comprises the following steps of: preparing materials, compositing, hardening, taking a mold, after painting coating, and closing a box for pouring. According to the technical scheme of the technology, the alkaline phenolic resin sand serves as surface layer sand, the water-glass sand serves as back layer sand, the advantages of the two molding materials are combined, and the alkaline phenolic resin sand and the water-glass sand are uniformly solidified by adopting CO2; the combination of the two molding materials can improve the dimensional accuracy and surface smoothness of casts under the condition of limited cost increment; as the usage amount of the surface layer land is little, expensive sand regenerative facilities do not need to be purchased by enterprises, and the surface layer alkaline phenolic resin sand has good collapsibility, so that the labor intensity in links such as sand removal, soldering and polishing can be reduced, and the loss of auxiliary materials is reduced; and small and medium-sized enterprises which adopt CO2 for hardening water-glass sand in past can adopt the composite casting technology for producing casts with high quality requirements under the condition that no facilities are added.