Nitrogen fixation is one of the most essential processes in chemistry. Developing more effective nitrogen fixation systems to catalyze the reaction under mild conditions is one of the most attractive but long-standing challenges. In this work, by means of first-principles computations, we systematically investigated the catalytic performance of transition metal atom-anchored C₂N monolayer electrocatalysts (TM_x@C₂N, x = 1 or 2; TM = Ti, Mn, Fe, Co, Cu, Mo, Ru, Rh, Pd, Ag, Ir, Pt, or Au) for N₂ fixation. The natively N edged uniform holes could fix the TM atoms firmly in TM–N_x configurations, which is beneficial for the N₂ fixation. Among them, Mo₂@C₂N exhibits the best catalytic performance for the reduction of N₂ to NH₃ with a maximum free energy change of 0.41 eV and an energy barrier of 0.51 eV, indicating that Mo₂@C₂N is a promising catalyst with high catalytic activity for the reduction of N₂ to NH₃. We believe that this work could provide a new idea for the design of N₂ fixation catalysts and shed light on C₂N monolayers as excellent substrates for catalysis.