This study presents a comprehensive framework for understanding the User Experience (UX) of foldable smartphones. Through product research, user case studies, and expert evaluations, this study defines the UX of foldable smartphones and identifies three core elements unique to foldable smartphones: the hinge, the user, and the folding interface. Special emphasis is placed on the hinge’s design, which is essential for preserving screen continuity, enhancing device portability, and ensuring user comfort during the folding and unfolding process. Additionally, ergonomic compatibility and psychological acceptability are identified as crucial for the user. Key aspects such as state consistency, intuitive compatibility, proper feedback, and unique features of the interface design are highlighted for facilitating smooth transitions across different folding states. The study’s findings are validated through a case study examining the impact of hand size on UX, revealing that users with larger hands exert more pressure and experience greater fatigue. The proposed UX framework holds significant implications for driving innovation in foldable mobile device design, elevating the quality of user experience, and supporting the successful mainstream adoption of this emerging product category.