<p>Hydraulic asphalt concrete (HAC) has been increasingly employed as an appropriate impervious structure in hydraulic and hydropower engineering. However, asphalt mortar, usually seen as the matrix of HAC composite, is particularly prone to damage under combined stress and seepage interactions, and the mesoscale investigations on the damage-seepage coupling behavior of HAC under complex stress states remain limited. This research develops a numerical three-dimensional mesoscale model composed of asphalt mortar and polyhedral aggregate to investigate the stress-damage-seepage coupling behavior in HAC. In this model, asphalt mortar yields the viscoelastic continuum damage law and aggregate obeys the Mazars’ elastic-brittle damage law; simultaneously, the effective permeability coefficient of asphalt mortar is assumed to follow an exponential function of damage. The predicted deviatoric stress-strain and hydraulic gradient-seepage curves both are in good agreement with the reported experimental results, which shows the proposed model is valid and reasonable. The simulated results indicate that the damaged asphalt mortar can induce localized areas of high permeability, which in turn affects the overall impervious performance of HAC.</p>