In the early stage of oilfield development, the reservoir has a high formation pressure and the viscosity of oil is very high. So it is necessary to take some methods to decrease the viscosity of the heavy oil. It is well known that doping thin oil is a good method to decrease the viscosity. However determine a reasonable incorporation depth is very important in actual application. In order to get a good method to determine the reasonable incorporation depth, this paper establishes the pressure models and temperature models of wellbore flow. Meanwhile, the adequate method which is used to calculate the viscosity was chosen. Also the optimal design of incorporation depth takes into account of some factors, which is sensitive to the change of incorporation depth, such as threshold pressure, the temperature and pressure of wellbore, in the spot where has great flow resistance, and so on. What is more, this paper has put this method into practice. According to actual data of A well, which is located in west reservoir, it is convenience to give the best incorporation depth by this optimal design method.