The invention relates to a water plugging agent, containing 66 to 70 parts of slag powder, 3 to 7 parts of kaolin, 3 to 7 parts of calcium oxide, 8 to 12 parts of calcium carbonate superfine powder, 6 to 10 parts of calcium silicate, 1 to 3 parts of sodium silicate, 0.5 to 1.5 parts of polyanionic cellulose and 0.5 to 2 parts of short linter; when the plugging agent is extruded to a target layer,the plugging agent is changed from a suspension state which is easily mobile to an uncured viscous state after a pump is stopped, so that the plugging agent can be effectively resided on the plugged part without missing, thereby raising the successful rate of plugging; and after entering the formation, the plugging agent enters a highly permeable water layer, the short linter is combined with gelsubstance, i.e. the sodium silicate, a grid structure is formed to reduce the permeation rate of the highly permeable water layer, thereby the difference of interlayer permeation rates of the water layer can be better solved to ensure that the plugging radii of the entire water layer are consistent, and the defect of the conventional water plugging agent that short effective period is caused by large difference of the plugging radii of the water layers with different permeation rates is overcome; besides, the water plugging agent reacts with the formation under borehole temperature and pressure in order to be tightly glued with the formation as a whole, and the water plugging agent has high compressive strength and is capable of plugging the water layer effectively.