To bolster the reliability of the structural design for drainage asphalt pavements, we have autonomously devised a cross-sectional flow testing apparatus to mimic authentic rainfall conditions. This apparatus serves to gauge the drainage capacity of pavements by utilizing the intra-structural cross-section flow as an innovative evaluation index. Additionally, we establish regression models to correlate this index with the drainage layer thickness, road surface slope, and void ratio. Simultaneously, based on the actual conditions at the project site to estimate drainage asphalt pavement’s void ratio, and with the mix design, to ensure that the road performance meets specification requirements. The findings indicate that the intra-structural cross-section flow is positively correlated with the drainage layer thickness, road surface slope and void ratio. Considering the project’s actual conditions, it is recommended to maintain a void ratio of not less than 22%. Under the design gradation and the optimal oil-stone ratio, the compound high-viscosity modified asphalt mixture exhibits superior high-temperature performance, water stability, and permeability. Simultaneously, the intra-structural cross-sectional flow exhibits a strong correlation with the permeability coefficient. This correlation suggests its potential as a viable replacement for the permeability coefficient, emerging as a novel index for assessing the drainage capacity of the pavement.