Using corn stover as raw material, the adsorption mechanism of ammonium nitrogen by biochar prepared by different modification methods was studied. The biochar was characterized by Fourier transform infrared spectroscopy, surface-area analysis and scanning electron microscopy. The results showed that the adsorption of NH 4 + − N by different modified biochars confirmed the quasi-second-order kinetic equation ( R ² > 0.95, p ≤ 0.05), the adsorption isotherms of the Langmuir equation ( R ² ≥ 0.96, p ≤ 0.05). Δ G ^θ < 0, Δ H ^θ > 0 indicated that the adsorption of NH 4 + − N by different modified biochars was a spontaneous endothermic reaction. With the increase in adsorption temperature, the adsorption capacity of biochar to ammonium nitrogen increased gradually. The adsorption was monolayer adsorption and was controlled by a fast reaction. Both KOH and FeCl ₃ modified biochars significantly improved the adsorption capacity of NH 4 + − N , and the adsorption mechanism was different. The adsorption capacity of NH 4 + − N by FeCl ₃ modified biochars mainly increased the specific surface area and micropore volume. The adsorption of ammonium nitrogen after KOH modification primarily depended on the wealthy oxygen-containing functional groups. The adsorption effect of ammonium nitrogen modified by KOH was better than that of biochar modified by FeCl ₃ .