The　conversion　of　CO2　into　value-added　chemicals　through　the　electrocatalytic　CO2　reduction　reaction　(CO2RR)　is　considered　as　an　up-and-coming　way　to　facilitate　the　renewable　closed　carbon　cycle　and　energy　storage.　Therefore,　the　development　of　robust　and　low-cost　CO2RR　electrocatalysts　is　essential　but　challenging.　Herein,　a　Ni　single-atom　catalyst　(Ni-SAs-NC)　was　successfully　prepared　via　a　precursor　assembly　strategy　and　a　subsequent　one-step　pyrolysis　process.　Characterization　studies　demonstrate　that　abundant　Ni　single-atom　active　sites　with　the　coordination　structure　of　Ni-N-4　are　uniformly　dispersed　in　Ni-SAs-NC,　which　consequently　endow　it　with　outstanding　electrocatalytic　CO2RR　performance.　Strikingly,　the　as-prepared　Ni-SAs-NC　can　selectively　convert　CO2　into　CO　with　a　high　faradaic　efficiency　of　up　to　98%,　and　can　maintain　the　high　values　exceeding　90%　in　a　wide　potential　range　from　-0.6　to　-1.0　V.　This　work　provides　a　new　protocol　to　prepare　high-efficiency　but　low-cost　single-atom　electrocatalysts　toward　the　CO2RR　and　other　electrochemical　reactions.