Electrochemical nitrogen reduction reaction (eNRR) is one of the most important chemical reactions for the production of ammonia under ambient environment. However, the lack of in-depth understanding of the structure-activity relationship impedes the development of high-performance catalysts for ammonia production. Herein, the density functional theory (DFT) calculations are performed to reveal the structure–activity relationship for the single-atom catalysts (SACs) supported on g-C₃N₄, which is modified by molecular groups (i.e., H, O, and OH). The computational results demonstrate that the W-based SACs are beneficial to produce ammonia with a low limiting potential (U_L). Particularly, the W-OH@g-C₃N₄ catalyst exhibits an ultralow U_L of −0.22 V for eNRR. And the competitive eNRR selectivity can be identified by the dominant *N₂ adsorption free energy than that of *H. Our findings provide a theoretical basis for the synthesis of efficient catalysts to produce ammonia.