Abstract: The deep clay exhibits different mechanical properties under different stress levels. The available test results show that the compression index and internal friction angle of normally consolidated saturated deep clay at high pressure are lower than those at medium and low pressures. In order to reveal the intrinsic mechanism of the clay for the above mechanical change, the <>e – <> p curve of the clay at high pressure is firstly compared with that calculated by the existing method which can predict the <>e – <> p relationship at large pressure by use of the parameters obtained from experimental <>e – <> p data at low pressure based on the diffuse double-layer theory. It is seen that the calculated results underestimates the compression of the deep clay at high pressure. Unreasonable hypothesis that there is a logarithm relationship between the dimensionless potential function at the midpoint of two clay platelets and dimensionless distance function is discovered. The improved hypothesis and the relevant procedure are proposed. There is quite good agreement between the calculated and experimental <>e – <> p relationships at large pressure. And then, based on the elastic adhesive friction theory, the calculated results show that the friction coefficient of the clay at 0.2 MPa is 2 times the one at 1.6 MPa, which is almost consistent to the measured one.