The invention relates to a sealing material, and in particular relates to a water-swelling nitrile rubber and a preparation method thereof. The water-swelling nitrile rubber is prepared by the following raw materials in parts by weight: 100 parts of nitrile rubber, 6.5 parts of active agent, 1 part of antioxidant, 40 parts of reinforcing filler, 4 parts of vulcanization accelerator, 0.5 to 2.5 parts of sublimed sulfur, 30 to 70 parts of water-absorbent resin, 10 parts of water-absorbing agent, and 5 to 20 parts of compatibilizer, wherein the compatibilizer is a maleic anhydride/nitrile rubber graft product. Compared with the traditional method for preparing the water-swelling nitrile rubber, the matrix of the water-swelling nitrile rubber prepared by the method has strong interaction with a water-absorbent resin, the water-absorbent resin does not easily fall off, the absorbency is increased, and the mass loss rate of the water-absorbent resin is reduced, so that the object that the rubber matrix and the water-absorbent component are evenly mixed can be achieved, and thus the cost is saved, and the preparation method is simple.