A better understanding of the moisture flow in the pavement drainage base layer can be achieved by the permeability coefficient for unsaturated soils, which form this kind of layer. In response, an equation to predict unsaturated permeability coefficient was proposed based on the Hagen-Poiseuille equation and Darcy law for sandy soils. The similarity relation between pore size distribution and soil water retention curve was used to derive an integration equation based on the capillary theory. The proposed equation was then validated against both water retention and saturated-unsaturated permeability data for six sandy soils from the hydraulic property database (UNSODA), illustrating that the proposed equation is superior to other published equations for predicting the relative permeability coefficients during drying in the matric suction range lower than 1000 kPa. Finally, the application of the proposed equation was presented based on the evaluation of the time-to-drain on the quality analysis of the pavement drainage base layer. The research results can provide theoretical support for the design of pavement subsurface drainage system.