Abstract: High earth-core rockfill dams (ECRDs) are characterized by complex stress paths, wide confining pressure ranges, and markedly different mechanical behaviors between the rockfill shells and the central clay core, leading to highly complex mechanical responses during construction and reservoir impoundment. The widely used Duncan–Chang model often exhibits significant inaccuracies—particularly in predicting longitudinal displacements—while advanced elastoplastic models typically require numerous poorly defined parameters that are difficult to calibrate, hindering their practical application. In contrast, the UH model features physically meaningful parameters that can be determined from conventional triaxial tests, enhancing its practicality; however, its performance in 300-m-class earth-core rockfill dams has not been adequately evaluated. In this paper, a 3D UH model was integrated and implemented on the high-performance computing platform Geodyna 8.0. To account for the creep characteristics of rockfill materials, creep and its associated hardening effects were incorporated into the model, and numerical simulations were performed on the full construction and impoundment processes of the Lianghekou Project, China’s first 300-m-class earth-core rockfill dam. The simulation accurately captures the spatiotemporal evolution of deformations in both the rockfill shell and the clay core, reproducing settlement and longitudinal displacement distributions with high fidelity, and representing the mechanical behavior of rockfill and core soil under extreme stress conditions. The results demonstrate the validity and engineering applicability of the UH model formulation and its numerical implementation, thereby establishing it as a reliable tool for the numerical simulation and safety assessment of high earth-core rockfill dams.