Abstract: The mixture of KD18 viscous crude oils and associated water in Xintan Oilfield commonly is water -inoil emulsion, which bond water contains about 50%~60%. The viscosity reduction must be conducted due to the high apparent viscosity. In allusion to the properties of the mixture and the actualities of the mixture's transportation, a new method of viscosity reduction for this emulsion by chemicals adding to make its phase conversion, dispersion and emulsification is presented. Based on this method, some high-effective viscosity-reducing agents are developed. The study results show that the apparent viscosity of KD18 viscous crude oils at 50℃ and 51 s^-1 can be reduced from 7000~10000 mPa·s to less than 100 mPa·s and reducing rate of viscosity is more than 98%, which provides an effective way to heavy viscous water-in-oil emulsion in reducing its viscosity.