The synthesis of poly(thioether), a highly desired sulfur-containing polymer, is still a key challenge. Herein, we report a simple and facile approach to poly(thioether)s by closed-system one-pot reaction of carbonyl sulfide (COS) and epoxides. This route underwent the coupling reaction of COS with epoxides, followed by decarboxylative ring-opening polymerization (ROP) of the generated mixed cyclic thiocarbonates with releasing of CO2 and a little bit of COS. Organic base was used as catalyst and initiator in the two steps, respectively. The oxygen/sulfur exchange reaction was driven by successive regioselective elementary reactions and spontaneous releasing of CO2 (COS), leading to the sulfur atom of COS transferring to poly(thioether)s, which was well demonstrated by DFT studies. This work provides an easyto-handle, metal-free route to poly(thioether)s bearing diverse structures by using readily available chemicals.