Background: In the context of globalization, different cultures and societies share common interests and goals in environmental protection. With the continuous economic growth and the improvement of living standards, societal trends show a heightened focus on environmental protection and public health. Under China’s “Dual Carbon” strategy aimed at achieving carbon peaking and carbon neutrality, seeking more efficient and environmentally friendly thermal management solutions has become particularly urgent. Objectives and Significance: This study summarizes the advantages and limitations of personal thermal management and modular clothing by reviewing existing research on personal thermal management and modular design in outdoor sportswear. The research proposes a modular lightweight outdoor sportswear design focused on personal thermal management needs, aiming to explore the potential and innovative pathways of apparel design in promoting individual comfort, health, and environmental sustainability from the perspective of environmental protection and public health. Since different modules of the clothing can be replaced or combined individually, wearers can participate in the design process and customize their clothing based on personal preferences and functional needs, achieving personalization in design. At the same time, this modular approach extends the life span of the garment, reduces resource waste, and promotes environmental sustainability. Methods: The design theory of “function-behavior-structure” is introduced into the research and design of personal thermal management clothing in this study. User requirement data were collected through desktop research, questionnaires, and interviews. Based on the personal thermal management needs summarized from the data, the modular design division strategy was derived by combining clothing structure, resulting in a modular prototype suitable for the study’s objectives. This prototype aims to achieve regional thermal management and enhance clothing adaptability and functionality. A cross-disciplinary research approach was adopted, integrating theories from textile material science, ergonomics, apparel design, and information technology. Material experiments, structural experiments, comfort assessments, and comprehensive practice were conducted, followed by thermal management performance testing in a controlled environment. Results and Conclusions: This study advances the integration of wearable flexible thermal protection technology and sportswear by proposing an innovative design scheme that reduces energy consumption. Modular clothing design meets personalized thermal management needs in various scenarios, providing a reference for the apparel design industry, environmental protection, and the development of a low-carbon economy. This research not only demonstrates the potential of apparel design in meeting individualized needs but also highlights the importance of cross-cultural collaboration in promoting environmental protection and health in the context of globalization.