Abstract: In view of the fact that the current formula for the critical hydraulic gradient is not suitable to solve the critical hydraulic gradient of cohesive soil, assuming that the seepage failure mode of the cohesive soil foundation is cylindrical or inverted circular, based on the formula for calculating the critical hydraulic slope proposed by Terzaghi, the analytical formula for the critical hydraulic slope under two failure modes considering the influences of the shear strength of the soil is derived respectively.In addition, the critical hydraulic slope of sandy loam and loess under different working conditions is studied by using the self-made permeation failure instrument.The results show that the critical hydraulic slope decreases with the increase of the soil thickness and failure radius, and the maximum error of the test results and the corresponding formula is less than 16%.