Metal nanoparticles (NPs) have been commonly introduced onto flexible platforms for improving their exploitation. However, such an introducing process of NPs is hard to achieve, and additional dispersants and high-energy consumption are largely required in existing studies. In this study, a one-pot method was developed to synthesize Au NPs in a cellulose dope. The dissolved cellulose chains acted as a green reductant as well as a stabilizer. As the polysaccharide dope coagulated, a three-dimensional (3D) regenerated cellulose nanocomposite decorated with 0.29−1.07 wt % Au NPs was successfully synthesized. As supported by the "hot spot" effect among the Au NPs embedded in the 3D structure, the nanocomposite could act as a sensitive surface-enhanced Raman scattering (SERS) substrate. Accordingly, this study achieved an enhancement factor of 2.8 × 10 7 and a limit of detection of 10 −9 M when R6G was employed as a probe molecule. Moreover, as impacted by the porous structure of the SERS substrate, 2.5 mg/kg melamine in milk could be directly detected. Furthermore, organic contaminants were catalytically reduced, and the process of catalytic reduction underwent in situ SERS monitoring with excellent sensitivity.