The discovery of metals as catalytic centers for nitrogen reduction reactions (NRR) has stimulated great enthusiasm for single-atom catalysts. However, the poor activity and low selectivity of available single-atom catalysts (SACs) are far away from the industrial requirement. Through the first principles screening, the doping engineering can effectively regulate the NRR performance of β-Sb monolayer. Especially, the origin of activated N2 is discovered from the perspective of the electronic structure of the active center. Among the 24 transition metal dopants, [email protected] showed the best NRR catalytic performance with a low limiting potential. The [email protected] also could significantly inhibit hydrogen evolution reaction (HER) and achieve a high theoretical Faradaic efficiency of 100%. Our findings not only accelerate discovery of catalysts for ammonia synthesis but also contribute to further elucidate the structure-performance correlations.