Based on the hydrodynamics in porous medium and the permeability characteristics of ore - heap granular medium, the rule of solvent seepage in heap leaching and the mechanism of the change of stress field act on seepage field are analyzed. The partial differential equation for the unsteady - seepage in anisotropic granular medium is established and the relation of seepage varying with void ratio of granular medium is deduced. The conclusion can be drawn that stress field affects granular permeability by impacting its volumetric strain and void ratio and as a result affects the seepage field.