Abstract: The solution method of instability seepage flow with Threshold Pressure Gradient(TPG) consideration was extremely complex because the governing equation was non-linear and flow boundary was not constant. In order to simplify the solution calculation, pressure characteristic about instability seepage flow in low permeability reservoir was studied with the approximate equation of formation pressure. Furthermore, the movement rule of the flow boundary at different times was gotten with material balance approach and the Newton iteration calculation. Finally, the pressure characteristic of reservoir and well under the condition of invariable productivity was given with approximate solution and analytic solution separately. The study shows that the TPG resists the propagation of energy and migrate of fluid, consumes part of the formation energy, and is harmful to the well production.