Tungsten-promoted titania solid acid catalysts were synthesized by a hydrothermal method and used in the hydrolysis of waste bottle polyethylene terephthalate (PET) in supercritical CO₂. The structure of the catalytically active sites in this system was determined by XRD, Raman spectroscopy, and HR-TEM. The surface acidity and reduction properties were studied by NH₃-TPD, titration experiments, and H₂-TPR. The results indicated that the tungsten phase existed as surface WO_x species, and a direct relationship among the number of nanoclusters consisting of polytungstate species on the surface, the number of Brønsted acid sites, and the catalytic activity was discovered. Partial reduction of WO_x species in the presence of the ethylene glycol produced during hydrolysis was also observed, and the polytungstate species were easier to reduce with increased condensation. A mechanism was proposed to describe the hydrolysis in which water molecules and hydronium ions were carried by supercritical CO₂ and penetrated the swollen PET matrix, and the hydrolysis occurred preferentially in the amorphous region of the surface and bulk of the PET matrix. The results reported here may help to pave the way for the design of active, reusable tungsten-based solid acid catalysts and highly efficient reaction systems for the polyester hydrolysis.