The invention relates to the field of rubber preparation and in particular relates to a method for preparing water swellable acrylonitrile-butadiene rubber. The preparation method comprises the following steps: preparing lithium acrylate, namely adding acrylic acid into a 100ml beaker, dripping a lithium hydroxide solution into the beaker, continuously stirring in the dripping process, uniformly mixing acrylic acid mixed with the lithium hydroxide solution, stirring and reacting in an ice-water bath for half an hour to obtain lithium polyacrylate, adding the reaction product acrylic acid intoa vacuum drying oven at 80 DEG C, and drying for 8 hours for later use; and preparing the water swellable acrylonitrile-butadiene rubber, namely adding 100 parts of acrylonitrile-butadiene rubber intoan internal mixer for plastifying for 2 minutes, adding 30 parts of lithium polyacrylate, mixing for 3-5 minutes, adding 3 parts of an initiator, mixing for 2-3 minutes, adding a cross-linking agent,and continuously mixing for 5-7 minutes, wherein in the mixing process, the temperature is controlled to be 60 DEG C or below, and the rotating speed of a rotor is 32r/min. The method disclosed by the invention is simple in process flow and convenient to operate, the production cost can be greatly reduced, the production efficiency and product quality are improved, and the mechanical property andwater absorption rate of the product are obviously improved.