Electrochemical nitrate reduction reaction (NORR) holds promise for the management of wastewater contamination and synthesis of carbon−neutral ammonia (NH). However, high−quality catalysts with controllable reaction pathways and high activity and selectivity are still lacking. The emerging single atom alloys (SAAs) offer attractive possibilities in nitrate reduction due to their unique atomic and electronic structures. By high−throughput first−principles calculations, we explore the possible incorporation of a series of transition−metal alloyed Cu−based SAAs, referred to as TM/Cu(111), for NORR toward NH. A hierarchical four−step screening strategy have been employed to evaluate twenty−seven SAA catalysts yielding three alloying elements (Ti, Ni and Nb) with high catalytic activity and NORR selectivity. Finally, only Ni/Cu(111) possess the best activity among these three candidates because of its lowest limiting potential of − 0.29 V. After further analysis, we found that the adsorption free energy of *NO can be recognized as efficient descriptor to design and predict the NORR performance of SAA. Furthermore, the Cu−based SAAs were revealed to exhibit pH dependent properties, which influence the competition between the hydrogen evolution reaction (HER) and NORR. This work not only indicates the significant potential of SAA in electrocatalysis for NORR to NH, but also highlights the important influence of pH on the activity and selectivity of catalysts under reaction conditions.