This study aimed at deriving the cubic law of radial flow of smooth parallel plates and study its applicability. According to the order of magnitude relationship between the inertial force and the viscous force, the distribution state of the flow was divided into the viscous zone and the potential flow zone. In the “weak inertial band”, flow was completely in the viscous zone, the cubic law of radial flow was derived according to the N-S equation and the Laplace equation. In hydraulic testing, hydraulic pressures of parallel plates and regular teeth are 0.1, 0.2 and 0.3 MPa, respectively; proving the agreement between the actual flux of the parallel plates and the cubic law and the applicability of rough fracture surface to cubic law. When the hydraulic pressure is 0.1 MPa, the flux of the parallel plates agrees well with the cubic law. However, high head differences are prone to nonlinear flows, causing flow to deviate from the cubic law. The presence of roughness will destroy the continuity of the water flow and weaken the discharge capability. The test value of the regular teeth is 7.9%-14.6% lower than the predicted value.