Utilization of carbon disulfide (CS2), a low-cost and sulfur-rich one-carbon (C1) feedstock, for making sulfur-containing polymers is important for both polymer chemistry and applied environmental science. However, the production of well-defined sulfur-containing polymers from CS2 and epoxides remains a key challenge. Here, we report a robust coupling reaction of CS2 and epoxides for chemoselectively producing poly(thioether)s and carbonyl sulfide (COS) in one pot. The resultant polymer has exclusive thioether units and a thiol (-SH) terminal. This route takes advantage of oxygen/sulfur exchanges through two consecutive reactions, i.e., cycloaddition of CS2 to epoxides and dethiocarboxylative ring-opening polymerization of the generated cyclic dithiocarbonates. The process can be conducted in ambient air with several commercially available organic bases as catalysts (initiators) and extended to a variety of epoxides, yielding COS and diverse poly(thioether)s with tens of grams. This work furnishes an original and practical strategy for utilizing CS2 as a sulfur feedstock in polymer synthesis.