Corn stalks were modified by magnesium loading and an oxidation process and then characterized by a series of methods. The ammonium in wastewater showed high sorption efficiency onto the polymeric adsorbing material during the process, with a biochars dosage of 20 g/L for 8h. Mg2+ was the dominant cation exchanger on the modified corn stalk, and it played an important role in ammonium sorption. Large amounts of NH4+ were exchanged onto the corn stalk, and they formed strong complexes with oxygen-containing groups on the corn stalk surface through polar bonds, resulting in the removal of NH4+ from the solution. Na+ present in wastewater was the major influence on ammonium sorption onto the corn stalk.