Psychological factors play a crucial role in achieving weight loss. However, most studies on clothing design related to weight loss interventions did not focus on psychological factors. This research integrates psychological theories and fashion design to develop the Extended Functional, Expressive and Aesthetic Consumer Needs Model, which encompasses the Functional, Expressive and Aesthetic Consumer Needs Model, the Transtheoretical Model, the Theory of Planned Behavior, Reinforcement Theory, and Body Image. This model is utilized to promote the design of functional clothing, which aims to encourage healthy weight loss behavioral patterns among consumes by influencing various psychological factors, including cognitive, attitudinal, and emotional factors. This study employed a reverse engineering approach to interview 15 overweight or obese participants and propose design strategies for functional weight loss clothing. The survey focused on five dimensions: demographic data, behavior change stages, existing problems, triggers, and demands and preferences for functional weight loss clothing. The findings revealed users’ clothing demands and preferences from functional, aesthetic, and expressive perspectives. Attitude, subjective norms, perceived behavioral control, experiential avoidance, and body image influence users’ intentions for healthy weight loss. Based on these findings, corresponding design strategy recommendations were derived.