The invention relates to a method for reducing the viscosity of heavy oil by emulsification. The method comprises the following steps: mixing an agent A selected from one or more of distillate oil, light oil or medium oil from a refinery plant, with the heavy oil; adding an agent B into the mixture of the heavy oil and the agent A; and blending to form an oil-in-water emulsion, wherein the agent B consists of an anionic surfactant, a nonionic-anionic surfactant and water. The method obviates the use of alkali and has high mineral salt tolerance. Particularly, the method can be used for emulsifying heavy oil with the ultra-high viscosity of 500,000mPa.s at 50 DEG C at moderate cost, and the emulsified heavy oil has low viscosity.