The rockfill dam with longitudinal reinforcements is one of the emerging dam types in which reinforced structures significantly affect the stress-deformation behavior, seepage characteristics, and slope stability. In this study, a three-dimensional model of the rockfill dam with longitudinal reinforcements was established to investigate the effects of the elastic modulus of rockfill zones on dam stability. Based on the strength reduction method, the stress-deformation characteristics of the dam were quantitatively analyzed to reveal the evolution patterns of plastic zones and displacement response mechanisms under varying modulus conditions. The results demonstrated that as the elastic modulus of the rockfill zones increased, the overall deformation of the rockfill dam, the displacement and maximal principal stress of the core-wall all exhibited a decreasing trend. The equivalent plastic strain within the dam decreases from 1.23×10−3 to 7.44×10−4 with increasing elastic modulus. This confirmed that increasing the elastic modulus of rockfill zones could enhance the stability of dam. These findings provide valuable insights for reinforcement design and the safe operational management of rockfill dams.