A series of waterborne polyurethane containing maleimide group emulsion were successfully synthesized by the reaction of poly-caprolactone diol(PCL1000), isophorone diamine(IPDI), 1,4-butanediol(BDO), dimethylol propionic acid(DMPA), trimethylol propane(TMP) and N-(4-hydroxyphenyl) maleimide (4-HPM) as main raw materials. The surface sizing properties of the polyurethane emulsion were studied, and the synthetic conditions were optimized. The results show that the emulsion owns excellent sizing property when R is 1.4, w(COOH) is 1.4%, w(4-HPM) is 1.0%, and with 2% polymer emulsion addition. The papers' sizing value can reach 64s, tensile strength approaches 57 N?m /g and folding endurance 82 times.