The electrochemical reaction mechanism and electrocrystallization process of Cu on copper electrode in the eutectic NaCl-KCl-CuCl molten salt were investigated by means of cyclic voltammetry, chronopotentiometry and chronoamperometry technique at 710 ℃. The results show that the electrochemical reaction process of Cu is a quasi-reversible process mix-controlled by Cu⁺ diffusion rate and electron transport rate; the electrochemical reduction mechanism is Cu⁺+e→Cu; the electrocrystallization process of copper is an instantaneous hemispheroid three-dimensional nucleation process; the Cu⁺ diffusion coefficient is 4.3×10^-4~cm²•s^-1 under the experimental conditions.