Transition 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 N fixation on molybdenum nitride by using density functional theory (DFT) calculations. The activity and selectivity of eNRR on pristine (001) and (110) MoN 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 MoN surface were both active for eNRR while following different pathways in mechanism. Moreover, the eNRR catalytic performance of MoN 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 MoN 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.