<p>The volumetric expansion of steel slag during the hydration process often leads to interfacial cracking and structural damage, posing significant challenges to the service reliability of cement-stabilized steel slag (CSS). In this study, a composite surface modification using stearic acid (STA) and a silane coupling agent (SCA) was employed to enhance the volumetric stability and mechanical performance of CSS. Based on the degradation mechanism induced by steel slag expansion, a multiscale damage evolution model was developed. The hydration degree of free calcium oxide (f-CaO) over time was described using the Avrami equation, from which a hydration-induced expansion rate model was formulated. A quantitative relationship between critical expansion stress and tensile strength was established via the Eshelby equivalent inclusion model. Furthermore, the nonlinear deterioration of strength under expansion stress was captured using a Weibull distribution function, while the dynamic environmental response was incorporated through the Arrhenius equation and a humidity correction function. Correlation analysis between the model predictions and experimental results showed an excellent agreement, with a fitting coefficient of determination <i>R</i>² &gt; 0.98 for the expansion stage and <i>R</i>² &gt; 0.95 for the damage coefficient. These results demonstrate that the proposed model effectively characterizes the expansion and degradation behavior of CSS and the modified cement-stabilized steel slag (CSMS), providing a robust theoretical framework for the design of volumetric stability and durability evaluation of steel slag-based road base materials.</p>