This invention relates to a method for improving the dewatering effect of demulsifier. The used quantity ratio of conventional demulsifier to cationic starch is 1:0.1-1:1.5, they are added to the crude oil to be dewatered separately, then the oil is kept in 40-65 deg.C sonstant temp. water-bath for 15 min, it is taken out and shaked out for 100 times, it is placed in constant temp. water-bath andkept standstill, and the dewatering quantities at 5, 15 and 60 min are recorded. The mentioned cationic starch includes quaternary ammonium salt type cation benzyl starch ether (QBS), quaternary ammonium salt type cation ethylol starch (QHS) and quaternary ammonium salt type cationic starch (QS). This invention not only raises thedewatering speed of demulsifier, but also improves the quality of dewatered water.