A binary catalytic system comprising tetrahydroxydiboron and tetrabutylammonium iodide (TBAI) was used to catalyze the cycloaddition of carbon dioxide (CO2) with epoxides. The tetrahydroxydiboron catalyst (9 mol %), in combination with the use of TBAI (13.5 mol %) as a nucleophile, is capable of catalyzing the cycloaddition of CO2 with various terminal epoxides under room temperature and a CO2 balloon. In addition, a range of internal epoxides, including sterically hindered bicyclic epoxides and vegetable oil-based epoxides, were suitable for the catalytic system, affording a series of cyclic carbonates in moderate to high yields. The tetrahydroxydiboron/TBAI cooperative catalytic mechanism was elucidated using Fourier transform infrared spectroscopy, nuclear magnetic resonance, and electrospray ionization-high-resolution mass spectrometry. Results reveal that the tetrahydroxydiboron catalyst exhibits dual effects, activating both CO2 and epoxides; initially, it underwent the insertion of CO2 to form a boron-CO2 adduct and subsequently activated the epoxides through interaction of the B-O bond.