The structural evolution of local coordination environments of single-atom catalysts (SACs) under reaction conditions plays an important role in the catalytic performance of SACs. Using density functional theory calculations, the possible structural evolution of transition metal single atoms supported by B/N codoped-graphene (TM-B2N2/G) under nitrogen reduction reaction (NRR) conditions is explored and the catalytic performance based on reconstructed SACs is theoretically evaluated. A novel nitrogen adsorption mode on TM-B2N2/G is discovered and the protonation of one of the N atoms results in the TM atoms binding with three N atoms, among which one associates with two B atoms (TM-N3B2/G). It is suggested that the N3B2/G supported tungsten single atom (W-N3B2/G) exhibits excellent N2 activity with a limiting potential of -0.27 V and high ammonia selectivity. Electronic structure analysis indicates that the coordination of N3B2/G redistributes the charge density of central W, shifts its d band center upward and strengthens the interaction of W and the adsorbed nitrogen molecule, thereby endowing it with better NRR performance, compared with that supported by pyridine-3N-doped graphene and pyrrolic-3N-doped graphene.