Near-natural forest transformation offers a promising pathway for restoring degraded soils, but the mechanisms linking biomass allocation to soil health under this management strategy remain largely unresolved in cold-arid ecosystems. To address this gap, we introduced Picea crassifolia into monocultures of Larix principis-rupprechtii and Populus cathayana to establish mixed forests, and evaluated soil quality dynamics along a 40-year restoration chronosequence. Transformation significantly improved soil conditions compared with monocultures, with increases in soil organic carbon, nitrogen availability, macro-aggregate stability, and moisture. The soil quality index rose by 37.8 % during the restoration process, with the fastest recovery occurring in the middle stage and the strongest effects observed in the 0-40 cm soil layer. These results indicate that soil recovery is both stage-dependent and depth-specific. Biomass allocation was identified as the key mechanism regulating soil improvement. Tree biomass served as the dominant driver of soil quality, while understory biomass provided complementary contributions, and litter exerted only a minor effect. By clarifying how biomass distribution across vegetation layers shapes soil recovery in cold-arid forests, this study advances understanding of vegetation-soil feedbacks and provides a scientific basis for near-natural forest strategies that accelerate soil fertility restoration and sustain ecosystem services.