Abstract: A denitrification system for removing nitrates in a water body by electrocatalytic hydrogen evolution and catalytic hydrogenation is disclosed. The denitrification system includes an electrolytic cell and a three-electrode system inserted into the electrolytic cell. The electrolytic cell contains electrolyte solution, and the denitrification catalyst is dispersed and suspended in the electrolyte solution. The three-electrode system includes a working electrode, a counter electrode and a reference electrode. The counter electrode adopts Pt sheet, the working electrode adopts the catalyst Ni3S2—NF, and the preparation steps of the catalyst Ni3S2—NF are as follows: (i) cutting a certain size nickel mesh and cleaning it; (ii) adding the cleaned nickel mesh into thiourea solution; (iii) reacting under hydrothermal conditions, and washing and drying to obtain the catalyst Ni3S2—NF.