Single-atomic transition metal-nitrogen codoped carbon (M-N-C) are efficient substitute catalysts for noble metals to catalyze the electrochemical CO2 reduction reaction (CO2RR). However, the uncontrolled aggregations of metal and serious loss of nitrogen species constituting the M-Nx active sites are frequently observed in the commonly used pyrolysis procedure. Herein, single-atomic nickel (Ni)-based sheet-like electrocatalysts with abundant Ni-N4 active sites were created by using a novel ammonium chloride (NH4Cl)-assited pyrolysis method. Spherical aberration correction electron microscopy and X-ray absorption fine structure analysis clearly revealed that Ni species are atomically dispersed and anchored by N in Ni-N4 structure. The addition of NH4Cl optimized the mesopore size to 7–10 nm and increased the concentrations of pyridinic N (3.54 wt%) and Ni-N4 (3.33 wt%) species. The synergistic catalytic effect derived from Ni-N4 active sites and pyridinic N species achieved an outstanding CO2RR performance, presenting a high CO Faradaic efficiency (FECO) up to 98% and a large CO partial current density of 8.5 mA cm−2 at a low potential of -0.62 V vs. RHE. Particularly, the FECO maintains above 80% within a large potential range from -0.43 to -0.73 V vs. RHE. This work provides a practical and feasible approach to building highly active single-atomic catalysts for CO2 conversion systems.