Ammonium adsorption characteristics and factors were investigated by Langmuir adsorption isotherm with long-term potassium fertilizer treatment in paddy soil. The results indicated that ammonium adsorption in the sampled soils was physical adsorption controlled by concentration gradients and the diffusion process. The quantity of ammonium adsorption in soil increased but the increasing intensity decreased with the increase of ammonium concentration in adsorption solution. Ammonium adsorption in smaller soil aggregates was higher than in bigger ones. Compared with CK treatment, long-term potassium fertilizer application improved ammonium adsorption in paddy soil, possibly resulted from the increase of illite content and pH value of soil, and the lower content of soil-fixed ammonium.