Abstract: This study proposes a new method for in-depth analysis of the deformation mechanism during the construction and operation of rockfill dams, using a hierarchical multiscale computational approach that couples the Material Point Method (MPM) with the Discrete Element Method (DEM). As a critical hydraulic engineering structure, the deformation of rockfill dams involves complex interactions between particle-scale behavior and the overall response of the dam body. Understanding the micromechanical deformation mechanisms is crucial for the safety assessment, design, and construction of dams. To comprehensively capture the deformation characteristics of rockfill dams, this study discretizes the dam body into multiple representative volume elements (RVEs) and simulates the behavior at the particle scale, capturing the local stress and particle contact force variations within the dam body. By analyzing the spatial distribution patterns of stress and deformation at different construction stages, the study reveals the significant impact of water level changes on the deviatoric stress and particle contact behavior of the dam. At the completion stage, the maximum settlement feature points exhibit significant contact anisotropy, with denser force chains in the vertical direction, indicating higher efficiency of stress transmission in the vertical direction. As the water level rises, significant changes occur in the contact state and force chain distribution within the dam, with increased horizontal anisotropy at the maximum settlement point, enhancing the dam's resistance to water pressure. During the normal reservoir level stage, particle contact on the upstream side of the main rockfill area weakens, and the coordination number decreases. The multiscale method employed in this study overcomes the limitations of traditional analytical approaches, providing a new quantitative framework for the deformation prediction, long-term stability evaluation, and design optimization of rockfill dams.