The recovery of heavy oil from ultradeep wells is a technical challenge faced by the petroleum industry. One of the key issues is the flow assurance of heavy oil inside the wellbore and through the pipelines, i.e., effectively lifting heavy oil from the wellbore and subsequently transporting it smoothly through the pipelines. Traditional chemical methods for reducing the viscosity of heavy petroleum fluids adopt oil-soluble and water-soluble viscosity reducers, but these methods have their limitations. In this work, we develop a new composite viscosity reducer called SDG-2, which combines the advantages of oil soluble and water-soluble viscosity reducers. The molecular chain of this viscosity reducer is grafted with a high-carbon lipophilic polar group and a salt-tolerant hydrophilic group, providing the reducer with both dispersion and emulsification capabilities. An experimental test shows that, without water, the viscosity reducer SDG-2 can achieve a 50% degree of viscosity reduction (DVR) for the ultraheavy oils tested. A low oil/water interfacial tension (IFT, 0.41 mN.m(-1)) can be achieved when 0.15 wt % SDG-2 is added into the highly saline (2.26 x 10(5) mg.L-1) solution. This IFT is significantly lower than the 13.42 mN.m IFT obtained by a 0.60 wt % oil-soluble viscosity reducer, as well as the 8.21 mN.m IFT achieved by a 0.40 wt % water-soluble viscosity reducer. The emulsification capability of SDG-2 is also superior to that of a water-soluble viscosity reducer; oil-in-water emulsion can be obtained when the oil is mixed with 30% (v/v) water and 0.30 wt % SDG-2. Higher DVR values (99%) can be achieved for the Tahe (China), Canada, and Venezuela heavy oils when emulsification occurs. Field tests of the new composite viscosity reducer are conducted in the Tahe oilfield of Xinjiang Province in China. Results show that considerably less light oil is needed to dilute heavy oil to the desired viscosity when the new viscosity reducer is used. On average, a 22.5% increase in heavy oil production rate can be obtained.