The invention discloses application of a Pd/Fe3O4 catalyst in removing nitrate or nitrite from a water solution. Specifically, the Pd/Fe3O4 catalyst is put into the water solution containing nitrate or nitrite, one or both of hydrochloric acid and sodium hydroxide are employed to control the initial pH of the solution at 4-11, and then hydrogen is introduced into the solution to undergo a catalytic reduction reaction. In the catalytic reduction reaction, ferroferric oxide plays a role of reducing nitrate to nitrite, and simultaneously provides an active site for nitrite reduction, plays a catalytic reduction role in a wide pH range, and can make nitrite entirely reduced. With superparamagnetism, the adopted Pd/Fe3O4 catalyst has saturation magnetization up to 75.9emu/g, and has the characteristic of easy recycling. The main products of catalytic reduction of nitrate and nitrite are ammonium ions, which can be completely eliminated by a break point chlorination method.