Abstract: In the midstream basin of the Dadu River, a set of deep and thick gravel layer is widely distributed, which is characterized as complex composition, varied sedimentary environment and special engineering properties.The permeability of this gravel layer is one of the key problems restricting the infrastructure activities along the area.We deeply research this gravel layer from the aspects of permeability coefficient calculation, state of flow, relationship between the permeability and the sedimentary characteristics through the in-situ particle size distribution analysis and the indoor-outdoor permeability test.The results show that this set of gravel layer mainly consists of fractured stones and breccia soils, whose effective particle size and non-uniformity coefficient are larger.In the test, the water migrates in a laminar flow way under a small hydraulic gradient meeting the Darcy′s Law, while it displays laminar-turbulent transition under a large hydraulic gradient, during which the flow velocity and the hydraulic gradient satisfies the power function relation.A proposed empirical formula could be applied to estimate the permeability coefficients of the two stages.This conclusion can provide references for the permeability coefficient calculation and the permeability strength evaluation of the deep and thick gravel layer in the research area.