Antarctic krill (Euphausia superba) is a keystone species in the Southern Ocean polar food web, and climate-driven shifts in its distribution profoundly influence the structure and functioning of the entire marine ecosystem. While species distribution models (SDMs) are widely employed to project such changes, most overlook life-stage-specific responses to environmental stressors (e.g., ocean warming and sea-ice retreat) and the influence of complex physical-biological processes, leading to uncertain distribution estimates. To address these limitations, we developed stage-structured SDMs integrated with model-transferability analyses to explore the mechanisms underlying spatial distribution and niche dynamics. Our results revealed stark, stage-specific niche partitioning. Under changing conditions, larvae experienced a southward contraction of suitable habitat and a narrowing of niche breadth. Conversely, juvenile and adult habitats expanded, shifting northward and southward, respectively. Crucially, transferability analyses indicated that non-environmental drivers often outweighed pure environmental tracking in driving these spatial decouplings. By revealing how different life stages respond divergently to climate-induced environmental shifts, this study advances our mechanistic understanding of krill redistributions. These findings provide robust scientific support for spatially dynamic, stage-resolved ecological conservation and resource management strategies, ensuring the rational use of marine living resources in a changing Southern Ocean.