Electrochemical nitrate reduction reaction (NO₃RR) has great potential for ammonia (NH₃) synthesis benefiting from its environmental friendliness and sustainability. Cu-based alloys with elemental diversity and adsorption tunability are widely used as electrocatalyst to lower the reaction overpotential for NO₃RR catalysis. However, phase separation commonly found in alloys leads to uneven distribution of elements, which limits the possibility of further optimizing the catalytic activity. Herein, an electro-triggered Joule heating method, possessing unique superiority of flash heating and cooling that lead to well-dispersed nanoparticles and uniform mixing of various elements, was adopted to synthesize a single-phase CuNi nano-alloy catalyst evenly dispersed on carbon fiber paper, CFP-Cu₁Ni₁, which exhibited a more positive NO₃RR initial potential of 0.1 V versus reversible hydrogen electrode (vs. RHE) than that of pure copper nanoparticles at 10 mA·cm⁻² in 0.5 mol·L⁻¹ Na₂SO₄ + 0.1 mol·L⁻¹ KNO₃ solution. Importantly, CFP-Cu₁Ni₁ presented high electrocatalytic activity with a Faradaic efficiency of 95.7% and NH₃ yield rate of 180.58 μmol·h⁻¹·cm⁻² (2550 μmol·h⁻¹·mg_cat⁻¹) at −0.22 V vs. RHE. Theoretical calculations showed that alloying Cu with Ni into single-phase caused an upshift of its d-band center, which promoted the adsorption of NO₃⁻ and weakened the adsorption of NH₃. Moreover, the competitive adsorption of hydrogen ions was restrained until −0.24 V. This work offers a rational design concept with clear guidance for rapid synthesis of uniformly dispersed single-phase nano-alloy catalyst for efficient electrochemical NO₃RR toward ammonia.