Formate and CO are competing products in the two-electron CO 2 reduction reaction (2e CO 2 RR), and they are produced via *OCHO and *COOH intermediates, respectively. However, the factors governing CO/formate selectivity remain elusive, especially for metal-carbon-nitrogen (M-N-C) single-atom catalysts (SACs), most of which produce CO as their main product. Herein, we show computationally that the selectivity of M-N-C SACs is intrinsically associated with the CO 2 adsorption mode by using bismuth (Bi) nanosheets and the Bi-N-C SAC as model catalysts. According to our results, the Bi-N-C SAC exhibits a strong thermodynamic preference toward *OCHO, but under working potentials, CO 2 is preferentially chemisorbed first due to a charge accumulation effect, and subsequent protonation of chemisorbed CO 2 to *COOH is kinetically much more favorable than formation of *OCHO. Consequently, the Bi-N-C SAC preferentially produces CO rather than formate. In contrast, the physisorption preference of CO 2 on Bi nanosheets contributes to high formate selectivity. Remarkably, this CO 2 adsorption-based mechanism also applies to other typical M-N-C SACs. This work not only resolves a long-standing puzzle in M-N-C SACs, but also presents simple, solid criteria ( i.e. , CO 2 adsorption modes) for indicating CO/formate selectivity, which help strategic development of high-performance CO 2 RR catalysts.