To investigate two-phase fluid flow processes influenced by phase interference, pressure drop, fracture roughness and environmental stress, a nitrogen-water two-phase flow experiment was carried out on highly rough granite fractures in an experimental triaxial cell. Pressure was used to control the two-phase fluid in the fracture. The results show that each fluid phase has a separate flow channel, even through rock fractures of large roughness. Correlation of the superficial velocities of the two-phase fluids identifies the annular flow at a high pressure drop due to the high kinetic energy of the gas phase; however, annular flow transitioned to complex flow with increasing fracture roughness and confining pressure. The relative permeability of water is greater than that of gas. The sum of the relative permeabilities of the two phases is less than unity due to phase interference. With increasing pressure head, confining pressure, and fracture roughness, the relative permeability of water shows a general decreasing trend and the sum of relative permeability continuously reduced, demonstrating that the localized flow paths of the different phases changed and the phase interference increased. The experimental relative permeability of gas is greater here than that determined by the nonlinear viscous coupling model and Corey model, but less than the straight-line relative permeability model (X-model). Among them, the viscous coupling model provides the closest approximation, indicating that the physical process of two-phase flow through highly rough and tight rock fractures is more like that through a pipe, rather than through porous media and parallel-plate channels.