To investigate the evolution of mechanical properties of gravelly soil subgrade fillers under different relative densities, this study has conducted a series of laboratory experiments to analyze the coupling mechanism between undrained shear behaviour and particle breakage. Using gravelly soil material collected from a highway site, specimens with four relative densities were prepared. Consolidated undrained shear tests were performed with a large-scale triaxial shear apparatus, combined with quantitative analysis of particle breakage characteristics, to reveal the regulatory effects of confining pressure and density on mechanical behaviour. The results indicate that increased density shifts the stress-strain response from softening to hardening during undrained shear. In addition, the pore pressure accumulation rate is jointly controlled by density and confining pressure, with low-density specimens under low confining pressure exhibiting a surge in pore pressure ratio, reaching up to 0.92, which indicates potential liquefaction risks. The critical stress ratio shows a slight increase with density, attributed to fine particle filling and altered frictional properties caused by particle breakage. Particle breakage rate significantly increases with confining pressure and initial density, and a hyperbolic relationship exists between breakage rate and plastic work. In the initial stage of plastic work input, energy dissipation is dominated by angular particle fracture, transitioning to frictional dissipation as loading continues.