The cationic plugging agent of water base drilling fluid was prepared by emulsion polymerization with styrene (St), butyl acrylate (BA) and cationic polymerizable monomer. The structure characteristics of the plugging agent were analyzed by infrared spectrum. The result of soaking test with clay ball showed that cationic plugging agent emulsion with a mass fraction of 5.0% was equivalent in inhibiting clay hydration property of saturated brine. The effect of cationic plugging agent on the properties of CaCl2 free phase drilling fluid was evaluated. The results showed that the high temperature and high pressure filtration (120~130 ℃) could be controlled at about 10 mL for the non phase drilling fluid with CaCl2 content of 25.0% when the cationic plugging agent is 2.0%.