Abstract: This study investigated the mechanical properties of a traditional lime-soil mixture, with a lime-to-soil volume ratio of 3:7, improved by calcined oil shale residue(hereafter referred to as oil shale ash). Using oil shale ash, calcium hydroxide, and loess as raw materials, the optimal mixing ratio for improving the traditional 3:7 lime-soil mixture was determined using unconfined compression tests and scanning electron microscopy(SEM). The improvement mechanism was analyzed. Additionally, the stress-strain curve characteristics obtained from the unconfined compression tests were used to develop a statistical damage constitutive model, reflecting the nonlinear deformation behavior during the initial compaction stage, using a macro-and micro-scale analytical approach. The results demonstrate that incorporating oil shale ash enhances the compactness of the microstructure, brittleness, peak strength, and elastic modulus of the traditional lime-soil mixtures, although the improvement in residual strength is less pronounced. The proposed constitutive model effectively captures the behavior of the improved 3:7 lime-soil mixtures before peak strength. It was observed that higher cementation levels result in slower damage progression and smaller damage values. With increasing load, the damage variable eventually approaches an asymptotic value. These findings provide valuable data to support the further application of oil shale ash in improving loess.