Using the tight sandstone cores in the Northern Songliao Basin, an experiment is conducted to determine the pseudo-threshold pressure gradient of two-phase flow under different physical properties and different water saturation. On this basis, a mathematical method for characterizing the pseudo-threshold pressure gradient of two-phase flow with rock properties and water saturation is established. We combined a mathematical model of unsteady water flooding relative permeability with a variable starting pressure gradient, (derived from non-Darcy percolation theory) with the mathematical characterization method of pseudo-threshold pressure gradient. This established a new method for calculating oil-water relative permeability of tight sandstone reservoir. The calculation results of the new method are compared with the JBN method. The achievements show that in a tight sandstone reservoir, with an increase of water saturation, the two-phase flow starting pressure gradient increases first and then decreases, and there is a strong correlation between the maximum pseudo-threshold pressure gradient and the Kleinberg permeability; The two-phase relative permeability curves of the Northern Songliao Basin tight sandstone reservoir have the following characteristics: The two-phase seepage zone is narrow, water flooding efficiency is relatively low, and the water relative permeability is low, which indicates that water flooding development is difficult.