Electrochemical reduction of CO2 to value-added products is an effective approach to manage the global carbon balance. However, the lack of effective electrocatalyst for CO2 reduction process is a major obstacle for its development. Currently, constructing atomically dispersed non-precious metal electrocatalysts presents a promising way to build high-performance and cost-effective electrochemical CO2 reduction systems. Herein we demonstrate a novel strategy to realize the anchoring and stabilization of isolated Ni atoms in the nitrogen-doped ultrathin porous carbon nanosheets via a polydopamine-assisted g-C3N4 template method. Benefitting from the abundant atomic Ni sites and ultrahigh specific surface area of porous 2D supports (>1000 m2 g���1), the catalyst exhibits excellent activity for CO2 reduction with particularly high selectivity towards CO, achieving a faradaic efficiency of 96% at ���0.86 V (vs. RHE) with a current density of 26.4 mA cm���2 in 0.1 M KHCO3 solution.