Atomically dispersed Ni sites on N-doped carbon catalyst exhibits outstanding catalytic performance for the electrochemical CO reduction reaction (CORR). However, the roles of Ni single atom (Ni) and Ni nanoparticles (Ni NP) were not clear. Herein, a series of feasible nitrogen doped carbon encapsulated Ni catalysts with different Ni species composites were synthesized by a facile pyrolysis method for electrocatalytic CO to CO. The self-tandem catalytic mechanism between Ni NP and graphite carbon with Ni-N sites (Ni/Ni@GCN), mainly originates from their unique spatially confined structure. Specifically, Ni-N obtained by controllable carbothermic reaction is deemed to be the active center to capture CO and stabilize *CO intermediate. Meanwhile, HO is activated and dissociated to *H by Ni NP at the defect site of the carbon layer, which accelerates the formation of the key reaction intermediate *COOH of CORR. Due to the molecule-level confined space of carbon layer defects, the multiple HO dissociation was blocked and the *H+*H reaction was inhabited. It results in nearly all the *H was captured by *CO rather than forming H. The Faraday efficiency of CO(FE) can reach 98.4 % at -0.88 V vs. RHE and maintained 94 % at -200 mA cm. This work revealed a self-tandem mechanism and clarified the synergistic mechanism of Ni/Ni@GCN for CO electroreduction to CO, which provides valuable insights for designing and identifying the active sites on Ni-N catalysts.