In water-cut conventional heavy oil production process, oil and water exist synchronously in well hole and the viscosity of oil/water mixture cannot be tested accurately by conventional coaxial cylinder rotational viscometer. To solve this problem, the agitator is used to build the viscosity calculated mode. The effects of light oil proportion on viscosity of produced mixture have been studied through simulated experiment of well bore temperature field. The oil-water mixture will show the trends of formation of W/O emulsion when augmenting light oil proportion in high water cut well and its viscosity will emerge peak value. The viscosity reducing rate will reach over 99% or more when augmenting light oil mass of water-free heavy oil well, lower temperature will lead to better performance of viscosity decrease.