In low permeability reservoirs，it is essential to exactly know the rule of fluid flow in porous media during waterflooding development. The relative permeability curve may intuitively reflect the characteristics of the oil and water seepage in porous media. The study of factors affecting the relative permeability curve mainly focuses on the inherent properties of rock（wettability and pore structure），flow medium（oil and water viscosity ratio），water dynamic condition（displacement pressure gradient and velocity），etc. The impact of the formation pressure on the relative permeability curve in the low permeability reservoirs has seldom been studied. Through mercury-injection experiment，the decline process of the formation pressure was simulated，and the relative permeability curves in the low permeability reservoirs under different levels of formation pressure maintenance were established. The variation of the oil and water seepage in the low permeability reservoirs was analyzed. The results of experimental study show that when the heterogeneity of pore structure becomes stronger and the formation pressure goes down，the oil-phase relative permeability declines and the water-phase relative permeability rises，and also the isotonic point shifts to the left and the two-phase region turns narrow，resulting in the increase of residual oil saturation. That is，the stress sensitivity exists in the oil-water seepage in the low permeability reservoirs. Analyses have shown that the elastic or plastic deformation in the reservoir rocks is the root cause of the stress sensitivity of the oilwater seepage characteristics. Thus the development philosophy of the low permeability reservoir has been put forward：the lower the initial permeability of the reservoir rocks is，the earlier the waterflooding time should be.