The adsorption mechanism of rice husk biochar to ammonium nitrogen was investigated. The influence of biochar amount, particle size, original concentration and pH value of ammonium nitrogen solution and shaking time on the adsorption characteristics were explored. The results showed that the unit adsorption quantities of ammonium nitrogen were increased with the increase of original concentration and pH value of ammonium nitrogen solution. The adsorption rate reached the highest point within 60 min and then kept unchanged within 90 min. The adsorption equilibrium of ammonium nitrogen was gained within 1.5 h. The maximum adsorption for ammonium nitrogen solutions was 81.14 mg/kg. Additionally, the effect of particle size on the adsorption process was significant. High adsorption quantities of ammonium nitrogen were got with small particle size and the adsorption capacity of rice husk biochar reached the maximum with particle size of 0.25 mm. The adsorption mechanism was investigated using thermodynamic and kinetics analyses. The results indicated that the adsorption behavior of biochar for ammonium nitrogen was inhomogeneous and it belonged to one of polymolecular layer fitted by Freundlich model properly. Quasi-dynamic equations were matched well with adsorption process of biochar to ammonium nitrogen, which included the diffusion in the fluid film, shift inside the grains and surface adsorption process by physical adsorption.