Abstract: Against the backdrop of accelerating energy transition, industrial upgrading, and intensifying great-power competition, critical mineral supply chain security has evolved from a resource assurance issue into a national strategic concern. Constrained by resource endowment, environmental regulation, and geopolitics, the supply elasticity of land-based critical minerals is tightening, making deep-sea critical minerals an important option for diversifying resource sources. Focusing on how deep-sea critical mineral development affects critical mineral supply chain security, this paper adopts a whole-of-supply-chain perspective, examines exposure, sensitivity, and adaptive capacity, and incorporates the Pressure-State-Response framework. The results show that, at the upstream stage, deep-sea development can diversify resource sources and reduce dependence on a few land-based supplier countries, though this effect is mainly long term; at the midstream stage, if processing, metallurgical technologies, and environmental compliance systems do not improve in parallel, risks may shift from the resource end to the processing and institutional ends; at the downstream stage, the impact is transmitted mainly through supply expectations, price fluctuations, and firm decision-making; at the recycling stage, the uncertainty of deep-sea development further highlights the buffering role of recycling in stabilizing supply and hedging risks. Accordingly, deep-sea critical minerals should be incorporated into integrated land-sea planning and medium- to long-term resource strategies, while relevant authorities should advance deep-sea engineering validation, metallurgical technology R&D, rule-tracking and assessment, and recycling system development in parallel to promote coordinated progress in resource development, industrial collaboration, and risk hedging.