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.