Vibroflotation gravel piles have been gradually applied to the treatment of earth-rock dam foundation in recent years. In order to study the influence of the foundation reinforced by gravel piles on the stress and deformation of the dam, a weathered concrete diaphragm wall dam on the alluvium deposit foundation was used as the study subject, and its foundation was reinforced by vibroflotation gravel piles. By considering the rheological characteristics of the dam material, we used the nonlinear finite element method to calculate and analyze the stress deformation of the dam during the water storage period and the running period with the reinforced foundation of vobroflotation gravel piles and the unreinforced foundation. By comparing the results of the analysis, we summanized the influence of the vibroflotation gravel pile reinforcement on the stress and deformation of the dam body and the diaphragm wall. The results showed that with the reinforcement of gravel piles, after the normal operation of 1 year, 5 years, and 10 years, the stress and deformation of the dam body and diaphragm wall on reinforced foundation were smaller than those on unreinforced foundation during water storage period, and the reduction of deformation of the reinforced foundation was relatively larger in magnitude. It can be seen that the reinforcement of gravel piles can effectively improve the stress and deformation of the dam body and the diaphragm wall, which is conducive to maintaining stable operation of the dam.