Single atom catalysts (SACs) exhibit the flexible coordinationstructure of the active site and high utilization of activeatoms, making them promising candidates for nitrogenreduction reaction (NRR) under ambient conditions. Bythe aid of first-principles calculations based on DFT, wehavesystematically explored the NRR catalytic behavior ofthirteen 4d- and 5d-transition metal atoms anchored on 2Dporous graphite carbon nitride C5N2. With high selectivityand outstanding activity, Zr, Nb, Mo, Ta, W and Re-dopedC5N2 are identified as potential nominees for NRR. Particularly,Mo@C5N2 possesses an impressive low limiting potentialof -0.39 V (corresponding to a very low temperatureand atmospheric pressure), featuring the potential determiningstep involving *N-N transitions to *N-NH via thedistal path. The catalytic performance of TM@C5N2 canbe well characterized by the adsorption strength of intermediate*N2H. Moreover, there exists a volcanic relationshipbetween the catalytic property UL and the structure descriptorΨ, which validates the robustness and universalityof Ψ, combined with our previous study. This work sheds light on the design of SACs with eminent NRR performance.