Single-atom catalysts (SACs) have been rapidly rising as emerging materials in the field of energy conversion, especially for CO2 reduction reaction. However, the selectivity and running current are still beyond practical applications. Herein, we report new unsaturated SACs with CO2 to CO selectivity of nearly 100% at 51 mA cm−2, and 91% at 100 mA cm−2. Such unsaturated SACs (M-Nx, M = Ni, Fe and Co, x < 4) are rationally prepared through a novel CO2-to-carbon process in large quantity and confirmed by X-ray absorption spectroscopy. As electrocatalysts for CO2 reduction, unsaturated Ni–N2 centered SACs exhibit outstanding activity for CO2 reduction, outperforming state-of-the-art unsaturated SACs. Operando X-ray absorption spectroscopy and theoretical calculation reveal that such unsaturated Ni sites with rich vacancies are favorable for production of more unpaired 3d electrons, and consequently reduce the free energy for COOH* formation, therefore boosting CO2 reduction performance. Not only does this work provide a new method towards unsaturated SACs in large quantity, but also contributes fundamental understanding of the unsaturated single-atom sites in electrochemical catalysis.