Abstract: Polymer flooding technology is widely used in oil exploitation.People have done a lot of research on ground process flow and formation seepage, but there are few researches on downhole injection process.The injection pipe diameter has an important influence on the downhole flow of the polymer solution.In this paper, based on the laminar flow theory of power-law fluid tubing flow, the parameters of flow regime, apparent viscosity and wall strain velocity of different concentrations of polymer solution under different daily injection rates are studied.The research shows that with the decrease of the pipe diameter, the Re value of the solution increases slowly and belongs to laminar flow; when the pipe diameter decreases to a certain extent, the Re value increases sharply, and belongs to the transition from laminar flow to turbulent flow; the pipe diameter continues to decrease, and the Re value rises rapidly and becomes completely turbulent.As the diameter of the pipe changes, the Re value will "mutate", and the smaller the daily injection rate is, the more obvious the "mutation" phenomenon is.The apparent viscosity of the polymer solution will increase with the increase of the diameter of the injection pipe, but the rate of increase will gradually decrease; it will decrease with the increase of the flow rate, and the rate of decrease will gradually increase.The wall strain velocity of the solution flow increases with the decrease of the diameter of the injection pipe, and the rate of increase gradually increases.The reasonable wall strain velocity should be maintained by the preferred injection pipe diameter to reduce the viscosity loss of the polymer solution.The research results in this paper have a guiding significance for the understanding of the tubing size design and viscosity changing rule.