Ring-opening polymerization of cyclic sulfur compounds is one of the important methods to synthesize sulfur-containing polymers. However, in traditional anionic polymerization techniques, the high reactivity of sulfur-centered anions tends to induce chain transfer side reactions, leading to products with broad molecular mass distributions and structural variations. These issues severely restrict the precise control over polymer chain structure and compromise the controllability and reproducibility of material properties. To solve this problem, this study presents a synergistic catalytic system that combines triethylborane (Et₃B), phosphazene base ^tBuP₁ and a thiol initiator to facilitate efficient and controlled anionic ring-opening polymerization of propylene sulfide. Experimental results show that adding Et₃B at 0 ℃ effectively blocks the chain transfer reaction of sulfur anions to monomers and prevents the formation of disulfide bonds. Density functional theory calculations confirm that Et₃B stabilizes the active sulfur-centered anion intermediates through strong B-S coordination, significantly reducing their nucleophilic reactivity and enabling precise control over the polymerization process. Based on these findings, a “one-pot, two-step” strategy for efficient synthesis of sulfur-rich polymers is developed. By using the dual-component catalytic system, an alternating copolymer of carbon disulfide and propylene sulfide can be synthesized as a macro-chain transfer agent, followed by the direct addition of styrene monomer to successfully create a well-defined polystyrene-co-poly (carbon disulfide-alt-propylene sulfide) terpolymer without isolating the intermediate. This work presents a novel pathway for the precise synthesis of sulfur-rich polymers.