Tight gas reservoirs generally produce water, which significantly impacts gas reservoir productivity. The influence of water on gas flow capacity is usually represented by relative permeability curves. In this paper, differences in relative permeability curves under various displacement pressures are analyzed through laboratory experiments, and the influence of gas reservoir pressure on the variation of relative permeability curves and productivity is discussed. Firstly, the steady-state method is adopted to test the gas–water relative permeability curves under different displacement pressure differences. Based on the analysis of the variation pattern of relative permeability curves and the microscopic flowing mechanism, the mechanism of water phase interference is further obtained. Secondly, a productivity model for fractured vertical wells considering changes in relative permeability curves is established to explore the impact of water phase interference on gas well productivity. The study finds that as the displacement pressure difference increases, the saturation of irreducible water and residual gas decreases, while the curvature of the relative permeability curve increases. The mechanism of water phase interference is that changes in the displacement pressure difference lead to variations in the flowable space and movable water saturation, which in turn cause corresponding changes in the effective permeability and relative permeability of the gas phase. In terms of gas well productivity, a threshold of production pressure difference exists. Beyond this value, the negative impact of water phase flow becomes more significant than the positive effect of increasing pressure difference, resulting in a decreased productivity as the production pressure difference increases. Consequently, a “turnaround” phenomenon appears at the end of the IPR curve. Moreover, different reservoirs vary in the severity and ease of water phase interference. The mechanistic study can guide the development of tight gas reservoirs and reduce the influence of the water phase.