Single-atom catalysts (SACs) have recently emerged as stars in boosting the synthesis of NH₃ from N₂, as the catalytic performance of the supported single atoms can be modulated by their coordination environment. In this work, we propose a new strategy, based on comprehensive density functional theory calculations, whereby the coordination environment of a single Mo atom can be tuned by a central heteroatom (X = Fe, Co, Ni, Cu, Zn, Ga, Ge, and As) in the Kegging-type polyoxometalate (POM, (XW₁₂O₄₀)ⁿ⁻) substrate to catalyze the electrochemical nitrogen reduction reactions (NRR). Firstly, we demonstrate that the single Mo atom binds strongly to the POM surface oxygen hollow sites without aggregation. Secondly, the adsorption of *N₂ on the POM-supported Mo atom is investigated and the reactivity is assessed by calculating the thermodynamics of the NRR. The results show that the POM (X = Co and As) supported Mo atom has high NRR activity with low limiting potentials. Finally, we reveal the origin of the NRR activity by analyzing the electronic structure. The results show that the charge on the O atoms of oxygen hollow sites is affected by the central heteroatom. Due to such effect, it can be found that more d electrons are transferred from Mo supported by POM (X = Co and As) to *N₂, thus the N≡N triple bond is activated. This strategy of coordination environment tuning proposed in this work provides a useful guide for the design of efficient catalysts for electrocatalysis.