The invention provides a sponge and a preparation method thereof. The sponge is prepared from the following raw materials in parts by mass: 45-60 parts of toluene diisocyanate, 25-35 parts of diisocyanate, 4-6 parts of water, 1-2 parts of a polymerization catalyst, 45-60 parts of polyether glycol, 3-4 parts of a flame retardant and 3-4 parts of a foaming agent. The preparation method comprises the following steps: (A) mixing and foaming toluene diisocyanate, 2.5-3 parts of water and 1-2 parts of the foaming agent, thereby obtaining polymeric sponge; (B) uniformly stirring diisocyanate, the rest water, stannous octoate, polyether glycol and the flame retardant, thereby obtaining a first mixture; and (C) fixing the polymeric sponge on the top of a mold, putting the first mixture into the mold, mixing and foaming the first mixture with the residual foaming agent to form an emulsion sponge, performing polymerization reaction on the emulsion sponge with the polymeric sponge, and forming, cooling and cutting, thereby obtaining a product. The preparation method has the advantages that the operation is simple, no adhesive is adopted for adhesion, and the like.