Belonging to the technical field of underground engineering plugging materials, the invention relates to a preparation method of an expanded polymer grouting material for high pressure and large flowkarst water inrush treatment. The method includes: 1) adding a crosslinking agent into acrylic acid, and then adding a macromolecular water-absorbent resin comonomer for polymerization reaction to obtain a precursor; 2) adding an initiator into the precursor obtained in step 1), raising the temperature and carrying out reaction to obtain hydrogel, and mincing the hydrogel to obtain primary macromolecular water-absorbent resin; 3) sprinkling a surface cross-linking agent to the surface of the primary macromolecular water-absorbent resin obtained in step 2), and then drying the product; and 4) preparing an inorganic water conducting agent into a suspension liquid, sprinkling the suspension liquid to the product obtained in step 3), and performing drying to obtain the expanded polymer grouting material. The polymer grouting material provided by the invention effectively solves the technical problem of difficult retention of grouting materials under a water inrush condition, and has the characteristics of green and no pollution, adjustable expansion time, and an expansion ratio up to 304 times.