To develop a bio-based polyol that combines low-carbon environmental benefits with high performance, a CO?-castor oil-based polyether polyol (CO?/CO-Polyol) was synthesized via bulk ring-opening polymerization using castor oil (CO), carbon dioxide (CO?), and propylene oxide (PO) as monomers. The chemical structure and thermal stability of the resulting polyol were characterized by hydroxyl value determination, gel permeation chromatography (GPC), Fourier-transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H-NMR), and thermogravimetric analysis (TGA). Subsequently, a series of polyurethane hot melt adhesives (PUR) were prepared by reacting the polyol with diphenylmethane diisocyanate (MDI). The influence of different polyol formulations on the melt viscosity, open time, and adhesion performance of the resulting PUR adhesives was systematically investigated. The results show that the synthesized CO?/CO-Polyol has a molecular weight of 3428 and a carbonate bond molar frction of 10.21%. Due to the simultaneous introduction of carbonate groups and the long aliphatic chains of castor oil, the polyol exhibits significantly improved thermal decomposition temperatures compared to conventional PPG polyol and castor oil-based polyether polyol, with the temperature at 50% mass loss increased by 98.27 °C and 51.15 °C, respectively. Furthermore, the CO?/CO-Polyol-based PUR demonstrates a higher melt viscosity (7800 mPa·s at 100 °C) and a shorter open time (210 s) than its PPG-PUR and CO-Polyol-PUR counterparts. In terms of adhesion performance, the pure CO?/CO-Polyol-based PUR achieves an initial adhesion strength (1 h) of 3.15 MPa on aluminum/PC substrates, representing an increase of 125% and 49.29% compared to PPG-PUR and CO-Polyol-PUR, respectively. After 24 h of curing, the shear strength reaches 5.82 MPa, and remains at 3.91 MPa after exposure to high-temperature and high-humidity aging conditions (85 °C/85% RH for 168 h), indicating excellent resistance to hygrothermal aging. This performance enhancement is attributed to the synergistic effect of CO? and castor oi, the carbonate segments introduced by CO? copolymerization enhance polarity and chain rigidity, while the multifunctional structure of castor oil increases crosslinking density, collectively forming a crosslinked network with high cohesive strength.