Single-atom catalysts (SACs) especially supported on two-dimensional nitrogen-doped carbon substrate have been widely reported to be able to effectively promote electrocatalytic N₂ reduction reaction (eNRR). The precise design of single-metal-atom active site (SMAS) calls for fundamental understanding of its working mechanism for enhanced eNRR performance. Herein, by means of density functional theory calculations, we theoretically investigate the eNRR performance of nine prototypical SMAS, namely, MN₂B₂ (M: transition metals of IVB, VB and VIB groups) which comprises of asymmetric ligands of N₂B₂ embedded in defective BN nanosheet. Our results reveal the significant role of spin state of SMAS in tuning the potential-determining steps of eNRR, in which MN₂B₂ site with higher spin magnetic moment (μ) is beneficial to reducing limiting potentials (U_L) of eNRR. Specially, CrN₂B₂ (μ = 4μ_B), VN₂B₂ (μ = 3μ_B) and MoN₂B₂ (μ = 2μ_B) demonstrate high activity and selectivity to eNRR. The asymmetric ligands of N₂B₂ are deemed to be superior over mono-symmetric ligands. More importantly, our results demonstrate that breaking (or deviating) of the scaling relations between key N-containing intermediates (*N₂H/*N₂ and *NH₂/*N₂) on MN₂B₂ can be realized by enhancing spin state of SMAS which renders the active site a balanced N-affinity critical for efficient eNRR. This observation is validated by the calculated Sabatier volcano-shape relation between eNRR limiting potentials and N₂ adsorption energy. Our study develops the guidance for catalyst design to boost eNRR performance by tuning the spin state of an active site.