Electrochemical conversion of N2 to NH3 offers a clean and energy‐saving solution for artificial NH3 production, but requires cost‐effective, steady and highly efficient catalysts to promote N2 reduction reaction (NRR). Herein, CuO employed as a new non‐noble‐metal NRR catalyst was investigated both experimentally and theoretically. When supporting the CuO nanoparticles on reduced graphene oxide (RGO), it was demonstrated that the resulting CuO/RGO nanocomposite could effectively and robustly catalyze NRR under ambient conditions. At −0.75 V versus reversible hydrogen electrode, the CuO/RGO exhibited a high NH3 yield of 1.8×10−10 mol s−1 cm−2 and Faradaic efficiency of 3.9 %, along with the excellent selectivity and high stability. Density functional theory (DFT) calculations revealed that the “Suf‐end” was the most effective mode for N2 adsorption on catalytic Cu atoms. In NRR process, the alternating associative route was the preferable pathway with *N2→*NNH being the rate‐determining step. Sub or Suf? Suf CuO/RGO nanocomposite exhibited a high NH3 yield of 1.8×10−10 mol s−1 cm−2 and Faradaic efficiency of 3.9 % at −0.75 V versus reversible hydrogen electrode. Theoretical calculations revealed that the “Suf‐end” was the most effective mode for N2 adsorption on CuO and the *N2→*NNH is the rate‐determining step.