Developing highly active single-atom sites catalysts for electrochemical reduction of CO₂ is an effective and environmental-friendly strategy to promote carbon-neutral energy cycle and ameliorate global climate issues. Herein, we develop an atomically dispersed N, S co-coordinated bismuth atom sites catalyst (Bi-SAs-NS/C) via a cation and anion simultaneous diffusion strategy for electrocatalytic CO₂ reduction. In this strategy, the bonded Bi cation and S anion are simultaneously diffused into the nitrogen-doped carbon layer in the form of Bi₂S₃. Then Bi is captured by the abundant N-rich vacancies and S is bonded with carbons support at high temperature, formed the N, S co-coordinated Bi sites. Benefiting from the simultaneous diffusion of Bi and S, different electronegative N and S can be effectively co-coordinated with Bi, forming the uniform Bi-N₃S/C sites. The synthesized Bi-SAs-NS/C exhibits a high selectivity towards CO with over 88% Faradaic efficiency in a wide potential range, and achieves a maximum FE_CO of 98.3% at −0.8 V vs. RHE with a current density of 10.24 mA·cm⁻², which can keep constant with negligible degradation in 24 h continuous electrolysis. Experimental results and theoretical calculations reveal that the significantly improved catalytic performance of Bi-SAs-NS/C than Bi-SAs-N/C is ascribed to the replacement of one coordinated-N with low electronegative S in Bi-N₄C center, which can greatly reduce the energy barrier of the intermediate formation in rate-limiting step and increase the reaction kinetics. This work provides an effective strategy for rationally designing highly active single-atom sites catalysts for efficient electrocatalysis with optimized electronic structure.