Continuously and finely tuning the electronic structure of metal active sites is essential to maximum the nitrate reduction reaction (NO3−RR) performance towards ammonia (NH3) and elucidate the reaction mechanism. Here, we employ single atomic Cu-N-C as a model system and develop a robust strategy to finely tailor the electronic structure of Cu via engineering of both the first and second coordination shell (CS) with B atoms. It is found that the first and second CS modification of B induces two opposite effects: the first modification leads to tension strains of Cu-N bonds and the decreased valence state of Cu, whereas the second CS modification leads to compressive strains and the increased valence state. Thanks to the bidirectional regulatory mechanism induced by B, we continuously tune the electronic structure of Cu to reach the top of adsorption volcano curves, thereby concurrently reducing the energy barrier of the NO3−RR and water dissociation step. Consequently, the optimized Cu-N4B2 catalyst displays superior NO3−RR performance than other Cu-N-C catalysts. Clearly, this work offers a guideline to design efficient NO3−RR electrocatalysts via finely tuning metal electronic structures.