The fluid flow in ultra-low permeability-tight oil reservoirs does not conform to Darcy’s law，which cannot be determined by the conventional Johnson，Bossler，and Naumann（JBN）method for calculating two-phase relative permeability curves. Considering the pseudo threshold pressure gradient of two-phase flow，the empirical relation was obtained through statistics for processing the experimental data of low-velocity non-Darcy flow and acquiring more accurate relative permeability curves. This empirical relation was employed to calculate the pseudo threshold pressure gradient at any water cut of rock samples with different air permeability. According to the experimental results of two-phase steady flow in low-permeability rock samples，a hypothesis was proposed，in which the pseudo threshold pressure gradients of different phases in the two-phase flow were equal，and a two-phase low-velocity non-Darcy flow model was constructed. Based on this model，a method was developed to obtain the relative permeability curves of two-phase non-Darcy flow. The proposed method were compared with the JBN method by using the experimental data of the rock samples from Daqing Oilfield. The results demonstrate that the relative permeability of the oil phase and the water phase changes significantly after considering the influence from the pseudo threshold pressure gradient of the two-phase flow.