Heterogeneous metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have garnered considerable attention in the two-electron CO 2 reduction reaction (2e-CO 2 RR). Interestingly, almost M-N-C SACs mainly produce CO, while Sb is one of the few SACs reported so far that can produce HCOOH. Nevertheless, the underlying factors for different selectivities on Sb-N-C SAC remain controversial, and the lack of in-depth understanding of limiting factors hampers further regulations. Here, by using constant-potential first-principles calculations, we revealed that the high HCOOH selectivity of Sb-N-C SAC is mainly attributed to their weak charge accumulation ability. Remarkably, considering the highly tunable geometric structure of M-N-C SACs, we provide that Sb-N-C SAC with the SbN 3 S 1 center is a promising candidate for CO production. Our work provides the mechanism insight into 2e-CO 2 RR selectivity and further paves the way toward electrocatalyst regulation and design.