The invention discloses a formula of heat-preserving composite foam concrete, which comprises the following components based on parts by weight: 500-1000 parts of concrete, 500-1000 parts of admixture, 5-20 parts of additive, 1-30 parts of polyurethane grains, 5-20 parts of foaming agent and 500-900 parts of water. The preparation method the foam concrete comprises the following steps of: preparing foaming agent water solution, smashing the polyurethane into grains, and adjusting the sizes of the grains as required, wherein the diameters of the grains are 1-3mm; adding the polyurethane grains into the concrete, the residual water after preparing the foaming agent water solution, the additive and the admixture, and evenly stirring to obtain slurry; mixing and stirring the slurry with the foam prepared by the foaming agent water solution; and maintaining, pouring and shaping into the cast-in-situ foam concrete. The invention solves the problems of the coordination between the intensity of the foam concrete and the heat-conducting property and the treatment of the waste polyurethane. The foam concrete can guarantee the intensity and has better heat-insulating effect.