The emulsification of heavy oil and water into a W/O emulsion would increase the viscosity of the W/O emulsion and reduce the mobility of heavy oil. As we know, addition of a viscosity reducer such as water-soluble surfactants could change the status of emulsion and improve the mobility of heavy oil. Although up to 90% viscosity reduction could be achieved in laboratory experiments, field improvement of the heavy oil recovery is not satisfactory with addition of a viscosity reducer. Actually, it is related to the difference of the mixing process of water-soluble viscosity reducer, water, and heavy oil between lab experiments and subsurface flow in the formation. The viscosity of heavy oil is significantly affected by the temperature and emulsification. As for the heavy oil sampled from Bohai Oilfield, its viscosity decreases by 95% when the temperature increases from SO to 100 degrees C. At 50 degrees C, the viscosity of the W/O emulsion with 70% water content is up to 21.05 times that of the dehydrated heavy oil. In the emulsification process of the W/O emulsion, water molecules disperse into heavy oil and agglomerate together to form water droplets, and the number of water droplets increases when the water content is increased. When we add some amount of a water-soluble viscosity reducer to the W/O emulsion, the re-emulsification process or the way of addition of the water-soluble viscosity reducer has a great effect on the variation of the state and viscosity. When heavy oil is mixed with 30% content of water-soluble viscosity reducer solution (the reducer content is 1% of the emulsion), up to 98% reduction in the viscosity of heavy oil could be obtained. However, when the W/O emulsion with 30% water content is mixed with 1% water-soluble viscosity reducer, only 4% reduction in the viscosity of emulsion is observed. In the re emulsification process of water in a W/O emulsion with addition of a reducer, pieces of heavy oil containing a number of water droplets are separated by the reducer solution and a W/O/W emulsion is generated. The pieces of heavy oil containing water droplets in such a W/O/W emulsion are much larger than the heavy oil droplets in an O/W emulsion. As the heavy oil in the reservoir is in the state of a W/O emulsion, a W/O/W emulsion with large heavy oil pieces containing water droplets will be generated when a water-soluble viscosity reducer solution is injected, and the resulted viscosity reduction is much lower than expected based on conventional lab experiments. Consequently, the experiments related to heavy oil with addition of a water-soluble viscosity reducer should be carried out with the W/O emulsion.