The pressure gradient exists in the oil-water two-phase percolation in tight and low-permeability reservoirs, which makes it very difficult to drive the remaining oil and residual oil in the matrix, and the water flooding production effect is generally poor. In this paper, the threshold pressure gradient and critical pressure gradient of water flooding with different permeability were experimentally measured, and the relationship between average water saturation and critical pressure gradient of water flooding during one-dimensional water flooding was analyzed. Besides, the formation reason of critical pressure gradient of water flooding oil was analyzed. The results show that the critical pressure gradient of water flooding increases first and then decreases with the increase of the average water saturation, and reaches a maximum value when the oil-water front is near the outlet end. The relationship between the maximum critical pressure gradient of water flooding and core gas permeability is a good power function, and the lower the permeability, the faster the maximum value of the critical pressure gradient of water flooding increases. It can be seen that the two-phase threshold pressure gradient first increases and then decreases with the migration of oil-water leading edge during water drive in tight and low-permeability reservoirs, and the capillary force between oil and water is the main reason for the formation of the two-phase starting pressure gradient.