Abstract: Based on the types of residual oil in pore media after water flooding and the viscoelasticity of polymer fluid in reservoir pore, undulating tube model which has the contraction-expansion cyclical process and upper-converted Maxwell model which is generally applied to numerical calculation of viscoelastir fluid areselected. The mathematics model made up of continuity equation,
equation of motion, UCM constitutive equation, stream function,vorticity equation as well as the boundary conditions are established. By effective numerical calculation, the mechanism of improving oil displacement efficiency ofymer fluid through mechanical analysis and theories analysis is:(1)the rolling action on oil film by first normal stress and shear stress from viscoelas-
tic polymer fluid is greater than the one by shear stress from Newtonian fluid with same viscosity, residual oil film can flow partly;(2)comparing polymer fluid with Newtonian fluid，the same stream line is not in the same position, and the swept vol-ume of viscoelastic polymer fluid is larger than Newtonian fluid
with same viscosity.

Key words: polymer, viscoelastisity, residual oil, dis-placement efficiency