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In many photocatalytic reaction paths, the breaking of the first CO bond in a CO2 molecule is often the key step that becomes the rate‐controlled reaction step. In this paper, a graphitic carbon nitride (g‐C3N4) supported nickel single‐atom catalyst (Ni@g‐C3N4) was successfully constructed, and the mechanism of CO2 catalytic reduction was systematically studied based on density functional theory (DFT). The introduction of nickel promotes the adsorption of small molecules, especially for the CO2 activation. According to density of states (DOS) and frontier orbital analysis, the photogenerated carriers tend to jump from nitrogen atoms to carbon atoms, forming an electron transfer in real space, after g‐C3N4 is excited by light. With the appearance of nickel‐doped levels, the DOS of Ni@g‐C3N4 is no longer symmetric with respect to the spin up and down, especially around the original band gap of g‐C3N4. Single‐atom nickel has abundant frontier orbitals and high activity and is a favourable place for chemical reactions. The presence of surface hydrogen can promote the recovery of CO2, and the energy barrier of Ni@g‐C3N4 with hydrogen is only 15% of the clean g‐C3N4 surface. This paper provides a new idea for the development of efficient single‐atom catalysts for CO2 reduction. [ABSTRACT FROM AUTHOR]