In order to study the permeability characteristics and permeability evolution model of gravel soil used in core wall of earth-rock dam, a self-made seepage-stress apparatus was used to conduct permeability test for gravel soil of the Changheba Hydropower Station in complex stress state. Test results show that complex stress has significant effect on permeability evolution of gravel soil, the relationship between seepage velocity and hydraulic gradient follows Darcy’s law within the range of the design hydraulic gradient, and seepage failure of soil samples does not occur. The results also show that under the same confining pressure, permeability coefficient of gravel soil decreases gradually as the partial stress and axial stress increases. There is a stepwise relationship between the axial strain and the stress acting time in the test process. As the partial stress grows, the axial strain increases with the seepage gradient. An empirical formula for relationship between the permeability coefficient and the stress function is established, which further reveals the permeability evolution mechanism of gravel soil under complex stress theoretically.