Abstract: Water-based polyurethane (WPU) is widely used due to its green and environmentally friendly characteristics. However, the introduction of hydrophilic groups has resulted in weaker thermal stability and mechanical properties, significantly limiting its application fields. In order to improve the performance of WPU, this study introduced cellulose nanofibers (CNF) as an enhancer into WPU. Using magnetic stirring, ultrasonic defoaming, and casting curing methods, WPU-based nanocomposite films with CNF mass concentrations ranging from 1% to 13% were prepared. Finally, conductive composite films were prepared using the method of doping with multi-walled carbon nanotubes (MWCNTs). The effects of CNF on the microstructure, thermal stability, and mechanical properties of WPU-based nanocomposite films were studied using SEM, FTIR, TGA, DSC, and mechanical testing. The sensing characteristics of the conductive composite films were tested using an LCR bridge. The results indicate that the introduction of CNF altered the microstructure of the WPU composite, creating strong hydrogen bonding within the material and enhancing its thermal stability. When the addition of CNF reaches 10%, the tensile strength of the CNF/WPU composite film is 4.10 MPa, with a breaking elongation of 470%. The elastic modulus and toughness are 1.47 MPa and 10.72 MJ·m⁻³, respectively. Subsequent cyclic tensile tests indicated that while the tensile strength of the material increased, it still maintained a high level of resilience and energy dissipation capability, demonstrating the reinforcing effect of CNF on the mechanical properties of the composite material. Finally, through the study of the sensing performance of the composite films after conductive doping, the application potential in the field of flexible wearable electronics is demonstrated, providing a new strategy for the design of multifunctional high-strength polymer elastomers for flexible wearable electronic devices.