In view of the problem of mining ultra-heavy oil is very difficult in Tahe oilfield, and traditional chemical methods for reducing viscosity of heavy petroleum fluids include oil-soluble and water-soluble viscosity reducers have their own limitations. In this work, a new effective composite viscosity reducer SDG-3 was developed, its performance of ability of dispersing heavy components, interfacial activity, emulsifying properties, viscosity reducing, salt and heat resistance, demulsification, et al, were evaluated. Experimental test show that, without the presence of water, the viscosity reducer SDG-3 with amounts of 1wt.% can provide a degree of viscosity reduction (DVR%) of 50% for the ultra-heavy oils tested, possessing the ability of dispersing heavy components in crude oil. A much lower oil-water interfacial tension (IFT) of 0.32 mN/m can be achieved when SDG-3 is added into water with a high salinity of 2.2×105 mg\L and heavy oil with viscosity of 1.81×105 mPa•s than the IFT of 27 mN/m without SDG-3. Emulsification capability of SDG-3 for ultra-heavy oil looks better than water-soluble viscosity reducer, the average droplet size of the oil in water (O/W) emulsion was 2.5μm. We can achieve more a degree of viscosity reduction (DVR) of 99% for Venezuela, CNOOC and Tahe (China) heavy oils when the emulsification occurs, which was better than traditional oil-soluble viscosity reducer whose DVR is 72.6%. SDG-3 has capacity of temperature-resistant 140 ◦C and salt-tolerant 2.2×105mg/L, respectively, having no influence on post-processing. Field tests of the new composite viscosity reducer are carried out in the Tahe oilfiled of Xinjiang province in China. The field tests show that 66.6 % less light oil is needed to dilute the heavy oil to the desired viscosity when the new viscosity reducer is used. On average, a 14.4% increase in the heavy-oil production rate can be obtained.