Electrocatalytic　reduction　of　nitrate　(NO3-　)　to　ammonium　(NH4+)　is　an　environment　friendly　approach　for　treating　nitrogen-containing　wastewater.　Consequently,　electrocatalysts　capable　of　effectively　and　selectively　reducing　NO3　-　to　NH4+　receives　growing　attention.　However,　Cu-based　catalysts　demonstrate　limited　efficiency　and　the　drawback　of　generating　toxic　NO2　-　though　it　is　prevalent　in　the　field　of　electrocatalytic　reduction　of　NO3　(NO3RR).　In　this　work,　a　CuNi　alloy　was　synthesized　on　a　Co　foil　through　electrodeposition　(marked　as　CuNi@Co)　and　its　electrocatalytic　performance　for　NO3RR　was　evaluated.　XRD,　EDS　mapping　and　XPS　characterization　indicate　CuNi　exists　with　alloy　state.　Electrocatalytic　experiments　demonstrate　that　CuNi@Co　exhibits　an　exceptional　FENH3　of　99.12%　at　-0.64　V　(vs.　RHE).　Importantly,　no　detrimental　NO2　-　generated　during　NO3RR　process　because　the　reduction　rate　from　NO2　-　to　NH4+　is　faster　than　that　from　NO3　-　to　NO2-　.　Mechanistic　analysis　suggests　that　Cu　serves　as　a　＇reservoir＇　to　provide　electrons　to　facilitate　the　reduction　of　NO3　-　to　NO2during　NO3RR　process　and　the　synergistic　effect　of　Ni　and　Cu　promotes　more　active　hydrogen　(H*)　to　participate　the　reduction　process　of　NO2　-　to　NH4+　not　H2　generation,　ultimately　improving　FENH3.　This　work　elucidates　the　role　of　electrons　and　H*　in　electrocatalytic　nitrate　reduction　to　ammonium　for　CuNi　alloy,　providing　new　insights　into　the　reduction　process　of　NO3RR　for　Cu-based　materials.