Abstract: Due to the effect of water level fluctuations, uneven deformation and crest cracks on the earth dams reinforced by the cut-off walls occurs, affecting its in-service performance. In this study, a physical model was constructed to monitor pore water pressure, earth pressure, and vertical displacement under eight cycles of water-level fluctuation, taking Wujing Reservoir dam as the prototype. The results show that the vertical deformation comprises the initial wetting-induced deformation and cumulative swelling–shrinkage deformation. With increasing cycles, the plastic deformation evolves through three distinct stages—rapid accumulation, slow accumulation, and stabilization, whereas elastic deformation remains unchanged. Uneven deformation between the upstream and downstream side is governed by the phreatic line amplitude and its overlying soil layer load. As the amplitude of the phreatic line increases, the vertical swelling-shrinkage deformation increases; as the load of the overlying soil layer increases, the upward swelling capacity of the soil decreases while the downward settlement increases. This study reveals the mechanism of uneven deformation of lateritic earth dams under water level fluctuations, helpful for its in-service performance evaluation.