The invention discloses a water shutoff agent and a preparation method thereof, which belong to the technical field of oilfield exploitation chemical preparation. According to the invention, after graphene powder, sodium periodate and nitric acid solution are stirred to react, filtration is carried out, filter residue is collected and washed, the filter residue, sphagnum teres herb and water are then mixed and ground, so that mixed slurry is obtained, the mixed slurry, glucose and ammonium tartrate are mixed for fermentation, the obtained fermentation product is dried, and reacts with methallyl alcohol and sulfuric acid solution, the obtained product is filtered, so that a filter cake, which is modified graphene, is obtained, the modified graphene then reacts with itaconic acid, a catalyst and the like, so that a graphene-modified polymer is obtained, the graphene-modified polymer, sodium fluorosilicate, an additive and the like are then mixed, so that the mixture is obtained, and finally, after the mixture is kneaded, dried and cooled, the water shutoff agent is obtained. The water shutoff agent obtained by the invention has a good water shutoff effect, and moreover, the water shutoff agent has excellent temperature-resistant and mineralization-resistant properties; after being injected into a stratum for use, the water shutoff agent cannot easily cause blockage, and can effectively increase the oil and gas recovery rate; and a good application prospect is realized.