Against the backdrop of a global environmental crisis, the fashion industry faces profound sustainability challenges. Remake fashion has emerged as a key solution, yet its theoretical research remains fragmented. Existing theories often focus on singular dimensions, such as material-centric (e.g., Circular Economy), socio-cultural-centric (e.g., Slow Fashion), or business & practice-centric (e.g., Participatory Design), lacking an integrated practical framework. This paper aims to fill this theoretical gap by systematically constructing and arguing for the Remake Design Theory. Through a critical review of existing theories, this study proposes a framework composed of three core pillars: (1) Remake Culture as the philosophical foundation and aesthetic driver; (2) the Remake Design Method as a structured innovation pathway, which includes a four-stage design process and five core principles; and (3) a Business Model as the market vehicle, emphasizing customization and customer participation. The contribution of this theory is its transcendence of the partiality of existing theories. It provides a complete action roadmap from cultural concept and design execution to commercial implementation. Furthermore, it offers differentiated practical guidelines for independent designers, niche Remake brands, and mainstream enterprises, providing a solution with both theoretical depth and practical applicability to advance Remake fashion from a niche practice to a sustainable industrial ecosystem.