Equivalent hydraulic aperture and hydraulic gradient are two important factors that significantly affect the permeability of rock mass fracture networks. A test model of fracture network is made and a high accuracy seepage testing system is established. The Navier-Stokes equations are solved to simulate the flow state of the fluid in the fracture network; and the influences of equivalent hydraulic aperture and hydraulic gradient on nonlinear seepage characteristics are studied. The results show that when the hydraulic gradient is smaller

the equivalent permeability holds constants

indicating that fluid flow is in the Darcy’s flow region; the flow and the pressure have linear relationship and the cubic law can be selected as the governing equation of fluid flow. In contrast

when the hydraulic gradient is higher

the equivalent permeability decreases dramatically with the increment of hydraulic gradient

and fluid flow is in the strong inertia region; the flow and pressure have a strong nonlinear relationship

which can be calculated by the Forchheimer equation. As the equivalent fracture aperture increases

the critical hydraulic gradient

which is utilized to characterize the onset of nonlinear flow in fractures

will decrease following a power law function. When the hydraulic gradient is smaller than the critical hydraulic gradient

the cubic law is selected as governing equation; when the hydraulic gradient is greater than the critical hydraulic gradient

the Forchheimer equation is selected as control equation; the parameters of a and b can be calculated according to the empirical formula. This study can propose useful suggestions to the determination of critical hydraulic gradient and the selection of governing equations when calculating fluid flow in fracture networks.