Abstract: Phase change materials (PCMs) are widely recognized as promising candidates for advanced thermal energy storage and temperature regulation. However, their practical application in smart wearable electronics and flexible devices remains challenging due to the intrinsic rigidity of the solid state and liquid leakage during phase transitions. Here, we report the design of a novel flexible phase change film constructed through crosslinking among hectorite, cellulose nanofibers (CNFs), and polyethylene glycol (PEG). The incorporation of hectorite not only strengthens the crosslinked network to improve mechanical robustness but also imparts excellent flame–retardant properties. The resulting flexible films exhibit a high latent heat of up to 100 J/g, superior tensile strength (16.84 MPa), high elastic modulus (1055.62 MPa), tunable phase transition temperatures in the range of ~20–65°C, and outstanding cycling stability with negligible latent heat loss after 50 heating–cooling cycles. Under extreme environmental conditions (−10 to 50°C) in a model house application, the films demonstrate remarkable thermal regulation capability, maintaining indoor temperature fluctuations within 4°C around 24°C despite external variations from −10 to 50°C. These results highlight the great potential of the synthesized flexible PCM films for wearable thermal management, flexible electronic devices, and sustainable building design. Keywords: Hectorite; Flexible PCM Film; Shape–Stability; High Tensile Strength; Thermal–Comfortable Application