Abstract: In view of nonlinear seepage characteristics during the flow of fluids in low-permeability reservoirs and the effect of the starting pressure gradient, the present paper established a mathematical model for the 3D oil-water phase seepage numerical simulation of reservoirs by integrating both nonlinear seepage and pseudo-linear seepage. The model for numerical simulation of low-permeability reservoirs was constituted by means of the fully implicit difference format and the preconditioned conjugated gradient method was applied to solve this model. The calculations of homogeneous reservoirs in the five-spot pattern and a certain block in the Daqing Oilfield indicated that the starting pressure gradient had a strong influence on the pressure distribution of low-permeability reservoirs. By the constant output, the greater the starting pressure gradient, the bigger the pressure variation and the larger the influenced area. Since the nonlinear flow formulation could describe both the flow behavior of fluids under a lower pressure gradient and the effect of the minimum starting pressure gradient in low-permeability reservoirs much better, the pressure calculated by the mathematical model using nonlinear flow formulation varied more gently than that using pseudo-linear flow formulation, thus, the model using nonlinear flow formulation is more practical for the exploration of low-permeability reservoirs.