Abstract: Utilizing nitrate (NO₃⁻) as the nitrogen source to produce ammonia can effectively remove NO₃⁻ pollutant while obtaining valuable ammonia (NH₃), and the understanding of the mechanisms is essential for the design of new catalysts. In this work, by using density functional theory (DFT) calculations, the electroreduction mechanisms of nitrate reduction reaction (NO₃RR) on transition metal single atom supported on 3N-coordinated N-doped graphene (TM/N₃-G) are systematically investigated. It is found that the protonation of *OH acts as the potential determing steps except for the traditionally considered *NO₃/*NO/*NO₂ protonation step and the desorption of water may play an important role for NO₃RR on some TM/N₃-G. By considering the stability of single-atom catalyst (SAC), the preferential adsorption of NO₃⁻ larger than H and H₂O, the limiting potential of whole NO₃RR, the selectivity toward NH₃, V (Mn, Os)/pyrrolic-N₃-G and Mn (Ru, Ir)/pyridinic-N₃-G are screened out as potential SACs for NO₃RR. This work provides an understanding of the NO₃RR mechanism and highlights several promising NO₃RR catalysts based on the TM/N₃-G system.