The invention discloses a metal element improved biochar-based nitrate radical adsorbent and a preparation method thereof. Straws are used as raw materials, and metal ions are impregnated, pyrolyzed and washed under certain conditions, so a metal ion improved cbiochar is prepared. According to the metal element improved biochar-based nitrate radical adsorbent and the preparation method thereof disclosed by the invention, by using adsorption research on nitrate radical waste water and comparing the adsorption effect of the biochar before and after improvement, the improved biochar prepared by the preparation method disclosed by the invention has good adsorption effect for potassium nitrate, wherein Mg<2+> improved wheat straw biochar has optimal adsorption effect for nitrate nitrogen; compared with the unmodified biochar, the Mg<2+> improved biochar prepared under the optimizing conditions has the advantages that the adsorption performance is improved by 10 times, and the adsorption equilibrium can be realized in a shorter time. The modified biochar adsorbent is a green adsorbent with application potential and can be used for effectively removing the nitrate nitrogen in a water body; besides, the effects of fixing carbon, reducing discharge and relieving the non-point source pollution are achieved.