The invention discloses a purifying agent for nitrates in water and application of the purifying agent, and aims to solve the technical problems that in an existing treatment technology, application is limited, secondary pollution is caused, and the nitrogen removal efficiency is low. The preparation method of the purifying agent for the nitrates in the water comprises the steps that biomass is ground and screened to prepare charcoal; iron-modified charcoal is prepared by adding a FeCl3 solution; the purifying agent for the nitrates in the water is prepared by adding acrylamide, N,N-methylenebisacrylamide and ammonium persulfate. The invention further provides a treatment method of the nitrates in a water system. The treatment method comprises the steps that pH and temperature of to-be-treated disposal sewerage are adjusted; the purifying agent for the nitrates in the water is added, and then the nitrates can be removed. The purifying agent for the nitrates in the water can be used for treating production wastewater, has the effect of preventing and controlling agricultural non-point source pollution and can be further used for treating nitrate pollution to drinking water, relieving harm, caused by the drinking water high in nitrate content, to the human health, preventing further deterioration of the water and providing an effective measure for protection of a water source.