The invention relates to a composite gel inhibitor for preventing and controlling spontaneous combustion of coal in a coal mine and a preparation method thereof. The composite gel inhibitor provided by the invention comprises the following components in percentage by weight: 10 to 65.0 percent of graft copolymer, 0 to 4.00 percent of flame retardant, 20 to 60.0 percent of water glass, and the balance of water. The graft copolymer containing a -COOH functional group is obtained by adopting a water soluble polymer and unsaturated carboxylic acid monomers to perform graft copolymerization, and the composite gel inhibitor for preventing and controlling the spontaneous combustion of the coal in the coal mine is obtained by adding the flame retardant and the water glass into the graft copolymer. The flame retardant provided by the invention has the advantages of excellent retarding effect, environmental friendliness and the like.