<p>Construction and demolition waste (CDW) is a low-carbon alternative to natural subgrade aggregates, which can alleviate the pressure of waste accumulation and resource exploitation. This study comprehensively analyzes the engineering properties, particle breakage mechanisms, and deformation behavior under environmental-loading effects of CDW as subgrade filler. CDW has both environmental and mechanical potential: it reduces solid waste landfilling and natural aggregate consumption, and its post-construction settlement after compaction is lower than that of traditional fillers. Even under the worst working conditions (brick aggregates with 96% compaction degree), its California bearing ratio (CBR)&#xa0;still reaches 21.6%. However, it has limitations such as a high abrasion value (up to 41%) and strong heterogeneity, requiring optimization through pretreatment (e.g., ball milling to remove mortar) and gradation control; CDW particle breakage occurs in three modes: fragmentation, abrasion, and grinding. Moderate breakage (controlling the proportion of 5–10&#xa0;mm particles to 25%–30%) can fill voids and improve shear strength, while excessive breakage leads to strength attenuation. Breakage can be quantified using indices like breakage rate&#xa0;(<i>B</i>_g) and fractal dimension; CDW breakage and deformation are affected by internal and external factors. Among external factors, moisture content exceeding the optimum moisture content (OMC)&#xa0;increases permanent deformation by approximately 20%; each freeze–thaw (FT)&#xa0;cycle reduces the resilient modulus by 15%–25% (the modulus stabilizes after 10 cycles). Deformation of CDW subgrades is controlled by the environment-loading coupling effect, and existing constitutive models cannot fully capture the complex interactive effects of multiple factors.</p>