Electrochemical reduction of CO2 to CO offers a promising strategy for regulating the global carbon cycle and providing feedstock for the chemical industry. Understanding the origin that determines the faradaic efficiency (FE) of reduction of CO2 to CO is critical for developing a highly efficient electrocatalyst. Here, by constructing a single-atom Ni catalyst on nitrogen-doped winged carbon nanofiber (NiSA-NWC), we find that the single-atom Ni catalyst possesses the maximum CO FE of over 95% at -1.6 V vs Ag/AgCl, which is about 30% higher than the standard Ni nanoparticles on the same support. The Tafel analysis reveals that the single-atom Ni catalyst has a preferred reduction of CO2 to CO and a slower rate for the hydrogen evolution reaction. We propose that the domination of singular Ni1+ electronic states and limited hydrogen atom adsorption sites on the single-atom Ni catalyst lead to the observed high FE for CO2 reduction to CO.