Abstract: Due to strong threshold pressure gradient and pressure sensitive effect Darcy’s Law cannot be used to describe fluid flow in low permeability reservoirs.To investigate the effect of effective stress on porosity and permeability as well as measuring the threshold pressure gradient in water flooding,natural cores which have strong pressure sensitivity were flooded using simulated formation water in the laboratory.Porosity and permeability under various effective stresses and threshold pressure gradient were obtained from the experiments.The mathematical model of threshold pressure gradient considering pressure sensitivity was established through the experimental data’s regression by using least square method.The concept of variable threshold pressure gradient was put forward in which threshold pressure gradient will increase as effective stress increases in low permeability reservoir.So,reasonable production pressure drop and the appropriate water flooding timing should be determined so as to prevent the impact of threshold pressure gradient on production.