Abstract: In the design of high rockfill dams with clay core wall on deep overburden, how to accurately predict the stress of concrete cut-off walls is the key scientific problem to be solved. An experimental study on the penetration of a cut-off wall into clay is carried out, and the deformation behavior of the clay at the top of the cut-off wall is obtained by means of the computed tomography. It is proposed that the flexible normal contact constitutive model should be adopted in the simulation of the normal contact between the top of the cut-off wall and the clay, and the finite element model is established to study the stress of the concrete cut-off wall under the rigid and flexible normal contacts. The results show that the top of the cut-off wall cuts the surrounding soil, and obvious penetration into soil displacement occurs. The normal hard contact constitutive model does not allow the contact surface to be embedded, so the restriction effect of the clay around the top of the cut-off wall is too strong, which leads to the high compressive stress at the top of the cut-off wall. The flexible normal contact constitutive model allows a certain degree of contact surface to be embedded and the embedding displacement of the contact surface can be interpreted as the actual penetration displacement. It can well simulate the punching failure phenomenon of cut-off walls penetrating into clay in practical engineering.