The development of electrochemical nitrate reduction reaction (NO₃RR) is a “two birds-one stone” method, which can not only remove NO₃⁻ pollutant, but also produce valuable ammonia (NH₃). However, a mechanistic understanding of the nitrate reduction process remains very limited. Herein, we highlighted a dissociative-associative mechanism for the NO₃RR, in which the N-O bond of nitrate is initially broken to form *O and *NO₂ intermediate adsorbed on two active sites (dissociation process) and then subsequently hydrogenated and reduced to ammonia (association process). By taking a series of diatomic site catalysts (CuTM/g-CN and CuTM/N₆C, TM= Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) as models, we systematically investigate the dissociative-associative mechanism for the NO₃RR and compared with the Cu-based single-atom catalysts which follows the traditional directly associative mechanism. Density functional theory (DFT) calculations show that dissociative-associative mechanism is energetically favorable on seven catalysts (CuTi/g-CN, CuV/g-CN, CuMn/g-CN, CuCo/g-CN, CuV/N₆C, CuCr/N₆C and CuFe/N₆C) with the significantly reduced limiting potential of − 0.14 V to − 0.47 V. Specifically, an efficiently screening strategy was proposed to determine the dissociative-associative or directly associative mechanism for NO₃RR. This work can provide useful guideline for the rational design and development of NO₃RR electrocatalysts.