The surface water is frequently contaminated by nitrogen which is mainly in the form of nitrate nowadays.This experiment used the methods of linear sweep voltammetry and potentiostatic electrolysis to study the reduction product and mechanism of nitrate on surface of copper electrode.Experimental results showed that the electroreduction product of nitrate strongly depends on the applied potential. The reduction of nitrate on copper electrode started from a potential ca. -0.6 V vs. Hg/HgO, followed by a reduction to nitrite between -0.8 V and -1.0 V is sub-nitrate.The reaction is identified as the rate-determining step of the nitrate electroreduction process.Between -1.0 V and -1.2 V, the nitrogen as the ideal product is produced, and finally converts ammonia at potential between -1.2 V and -1.4 V. The reaction rate increases and the selectivity to ammonia also rises with the decrease of electric potential.