In order to obtain the permeability of core wall mixed gravel under complex stress conditions, permeability tests on core wall mixed gravel are carried out by modified middle triaxial apparatus. Thereafter, the effects of different confining pressures, soil-rock ratios, and water head differences on the permeability coefficient of core wall mixed gravel are studied. The test results show that the larger confining pressure is, the smaller permeability coefficient of core wall mixed gravel is, that is, the relationship between them is shown as a negative exponential function. When the confining pressure increases to a certain value, the permeability coefficient almost keeps unchanged because the soil particles are difficult to be further compressed. In addition, the permeability coefficient has a negative correlation with the soil-rock ratio and it increases rapidly when the gravel content exceeds a certain value. Moreover, when the water head difference is small, it has little or no effect on the permeability coefficient.