AbstractTransition metal nitrides (TMNs)‐based materials have attracted increasing attention in electrochemical nitrogen reduction reaction (eNRR) because of their unique structures and inherent electronic properties. However, the eNRR mechanism on such nitrogen contained catalysts is still unclear, for example, which part of the catalyst act as the active sites, and how to achieve the optimal efficiency is also challenging. In this work, a comprehensive study was conducted to unravel the reaction mechanisms of N₂ fixation on molybdenum nitride by using density functional theory (DFT) calculations. The activity and selectivity of eNRR on pristine (001) and (110) Mo₅N₆ surfaces as well as few specific numbers of heteroatom‐anchored N‐terminated surfaces were all evaluated and compared. It was found that the Mo and N atoms on the pristine Mo₅N₆ surface were both active for eNRR while following different pathways in mechanism. Moreover, the eNRR catalytic performance of Mo₅N₆ could be further boosted by specific metal atoms anchoring, such as single atom, metal dimer, and heterodiatom pair. Finally, a full map of eNRR mechanism on pristine and metal atom‐decorated Mo₅N₆ surfaces was illustrated. This work not only provides a fundamental understanding of eNRR mechanism on TMNs based materials but also offers powerful strategies towards the rational design of efficient NRR electrocatalysts.