The discovery of metal-nitrogen centers as the active sites for electrolysis has aroused significant interest in utilizing single-atom catalysts for nitrogen reduction reaction (NRR). Properly designed nanostructured catalysts that strongly interact with nitrogen molecules (N₂) can promote adsorption and activation, thereby resulting in efficient catalysts with high stability, activity, and selectivity. In this study, using density functional theory calculations, we selected monolayer black phosphorus (BP) as the substrate and screened a series of single-atom transition metals confined in tri-coordinated and tetra-coordinated active centers (without and with N dopants) to electro-catalyze NRR. As a result, we have identified two promising candidates (Hf₁-N₁P₂-1 and Tc₁-N₄), which exhibit not only low overpotentials of 0.56 and 0.49 V but also high thermodynamic and electrochemical stability, as well as good selectivity towards NRR over the competing hydrogen evolution reaction. We also demonstrate the ability of Hf₁-N₁P₂-1 and Tc₁-N₄ to activate and hydrogenate N₂ by donating electrons and regulating charge transfer. This study not only predicts new BP-based promising catalysts but also provides guidance for the rational design of high-performance NRR electrocatalysts under ambient conditions.