<p>To systematically investigate the mechanical behaviors of rockfill materials under the combined influence of particle shape and sample size, a stochastic algorithm that incorporates three-dimensional shape parameters such as elongation, flatness, sphericity, and convexity, was employed to generate irregularly shaped rockfill particles. A triaxial numerical simulation method was proposed for rockfill materials, which comprehensively accounts for both particle shape and sample size. Discrete element numerical simulations of rockfill materials with varying particle shapes and sample sizes were conducted to investigate the effects of these two factors on the mechanical properties and nonlinear constitutive models from both macroscopic and microscopic perspectives. The evolution of microscopic characteristics including coordination number, Euler angles, and fabric anisotropy of rockfill bodies were examined, and the stress–strain and volumetric strain responses of various particle samples were analyzed, revealing the correlation mechanisms of particle shape and sample size on their macro- micro properties of rockfill materials. Based on these findings, a novel constitutive model that integrates both particle shape and sample size was presented, aiming to offer a more accurate and comprehensive theoretical framework for understanding the mechanical behavior of rockfill materials. </p> Graphical Abstract <p></p>