<p>The concrete face rockfill dam is widely used in the construction of pumped storage power stations. Significant daily water level fluctuations in pumped storage power stations result in slow-rate cyclic loading effects on the rockfill materials behind the slab, leading to particle breakage and accumulation of deformation. The purpose of this study is to explore the impact of slow-rate cyclic loading on the crushing behavior of individual rockfill grains at the particle scale. First, a serious of single-particle crushing tests were conducted to investigate the impacts of particle size, cyclic loading times and cyclic loading amplitude on particle crushing. The load–displacement curves and fragmentation patterns under different test conditions were further discussed. Additionally, the test results were analyzed using the Weibull statistical method, and the variations in Weibull characteristic strength under different test conditions were explored. Finally, a single-particle crushing model was proposed to reflect the size-dependent and cyclic loading-related characteristics of particle Weibull characteristic crushing strength. In the newly proposed model, the concept of fatigue-crack propagation was applied to tackle the strength degradation of rockfill grains under slow-rate cyclic loading effects, and the Weibull statistics was adopted to consider the influence of the random defects on the scale effect of crushing strength. The robust performance of the newly proposed model was verified by comparing the predicted results with statistical Weibull characteristic strength, with the coefficient of determination exceeding 0.92. This work provides a particle-scale perspective on the degradation behavior of rockfill under slow cyclic loading, while the research findings establish a connection between grain-scale phenomena and continuum-scale processes in rockfill materials under such conditions.</p>