The filter paper method discussed in this paper shows good sensitivity and reproducibility to unsaturated soil solut ion concentrat ion changes, so it can be used to obtain ion adsorption isotherms in unsaturated soils. It is also quite concordance with the centrifugal method and the ethyl alcohol extract ing method. NH₄⁺ 、K⁺ adsorption isotherms in unsaturated soil obtained with the filter paper method possess ordinary characteristics of ion adsorpt ion isotherms, and can be significantly fitted by Freundlich equation. With a certain equilibrium concentration, the adsorbed amount in solid phase and fitted parameters (k, n) of Freundlich equation increase with increasing solution/soil ratio, indicating that increasing solution/soil ratio can promote ion adsorption reaction in unsaturated soil.