The common deformation and failure mechanism of retaining wall at foot of earth-rock dam is mainly caused by insufficient bearing capacity of foundation, uneven settlement of foundation, temperature effect or insufficient anti-sliding stability. For a certain project, the retaining wall deformation characteristics are not consistent with the above causes after the drainage of the embankment foot of the earth-rock dam is changed to the retaining wall. The deformation is only visible at the expansion joints of the retaining wall, and the expansion joints are open, closed or misplaced. Through calculation and analysis, it is found that the expansion joint deformation is caused by the retaining wall horizontal displacement caused by the consolidation settlement of the dam, and the size of the deformation is related to the horizontal displacement difference between adjacent retaining walls. The larger the displacement difference is, the more serious the deformation is. At the same time, the horizontal displacement of retaining wall is related to the height of dam section, hydrogeologic conditions of dam foundation. Based on this, it is proposed that after the prism drainage of the downstream earth-rock dam is changed into gravity retaining wall, in addition to the design and calculation according to the standard requirements, the horizontal displacement difference between retaining walls should also be calculated, and the damage of retaining wall displacement and poor drainage to the downstream drainage system of the dam body should be considered. The research results have a good reference value for the correct analysis of deformation mechanism and anti-slip design of retaining wall at dam foot.