<p>Carbonaceous mudstone is widely utilized as subgrade filler; however, its application is often constrained by particle breakage, which adversely affects its mechanical properties and embankment performance. A systematic investigation is conducted on the influence of particle size, water content, and impact load on particle breakage through impact loading experiments conducted on four sample groups with uniform initial particle size distributions. The results reveal that increasing impact energy transitions particle size distributions from uniform to non-uniform, with this effect being more pronounced at higher water contents. Particle breakage intensifies with successive impact cycles, resulting in fractal distributions and higher fractal dimensions. Vertical strain and relative breakage rates exhibit a positive correlation with energy input, while volumetric deformation progresses through three distinct stages: compression, breakage, and stabilization. An energy dissipation model is developed to quantify energy variations during breakage, demonstrating a rapid increase in surface energy during initial impact cycles, followed by stabilization. Notably, larger particles accumulate greater surface energy under identical conditions. These findings offer a quantitative framework for understanding the dynamic behavior of carbonaceous mudstone and provide practical insights for optimizing its performance as subgrade filler.</p>