Abstract: The effective viscosity,viscoelastic property and molecular stagnant quantity of polymer fluid are all the function of flowing speed.This paper studies the rheology of polymer fluid in flowing through porous media,analyses the critical velocity Vcl in which the stretch elongation appears and the critical velocity Vc2 in which the expansion-flow appears.It thinks that there is a optimum oil-displacement velocity range in polymer drive based on the further research on the effect of flow velocity on the recovery factor.