The construction timing of concrete cut-off walls in the old reservoir dam heightening project has a great influence on its own stress and deformation, which may cause potential safety hazards to the anti-seepage system of the dam body. Taking a heightening project of an earth-rockfill dam as an example, the three-dimensional finite element method was used to simulate the heightening process of the dam, analyze and compare the stress and deformation characteristics of the cut-off wall under different construction schemes, and discuss the optimal construction timing of the cut-off wall. The calculation results show that under the conditions that the cut-off wall of the old dam is built when or before the heightened dam is filled to 215 m, the stress and deformation of the cut-off wall are lower, and during the completion period and the water storage period, the displacements of the cut-off wall along the river are about -1.56 cm and 4.91 cm, the maximum principal stresses are about 1.01 MPa and 1.67 MPa, and the minimum principal stresses are about -0.41 MPa and -0.66 MPa, respectively. The extreme values of the tangential displacement and relative settlement of the joint between the cut-off wall and the connecting plate are 4.84 mm and 8.11 mm, respectively, which can meet the requirements of safe operation. Compared with the cut-off wall constructed in the late stage of heightening dam filling, completing the construction of the cut-off wall before the filling of the heightening dam starts or before the filling reaches the same height as the old dam can improve the stress and deformation state of the cut-off wall and reduce the tangential displacement and relative settlement of the joint between the cut-off wall and the connecting plate.