CO2 and SO2 appear simultaneously in most industrial cases and have caused serious environmental and health issues, however, the simultaneous conversion of SO2 and CO2 has not been reported until now. This work describes an alternating terpolymerization of SO2/CO2 with cyclohexene oxide using SalenCr(III)Cl as catalyst and bis(triphenylphosphine)iminium chloride as cocatalyst, and a novel poly(cyclohexene carbonate sulfite) was obtained. The terpolymers were characterized by nuclear magnetic resonance spectroscopy, infrared spectroscopy, and gel permeation chromatography, and effects of the different reaction conditions on the structures of the terpolymers (carbonate content and sulfite content in the block copolymer) were investigated. In experiments, we found that SO2 not only acts as reactant, but also exhibits a nucleophilicity to promote the ability embedding of SO2 in the terpolymers. According to the study of mechanism that proposed by predecessors and experiment, the possible terpolymerization mechanisms are recommended. The results proved that the terpolymerization of CO2/SO2 with epoxides may be considered as an effective approach to simultaneously convert CO2 and SO2, and improve efficiency of the CO2/SO2 utilization.