Abstract: The concept of swelling force of expand soils was analyzed and it was pointed out that the swelling force described by Basic Nomenclature Standard of Geotechnical Engineering (GBPT 50279-98) was limit swelling force. The moistening trial to expansive soils sample was carried out and some valuable conclusion was drew that the moistening will come into being greater swelling forces at forepart and the inflexion comes forth to a certainty phase and then the relation approximates to linear,and that the relation is the non-linear negative correlativity of primary water content and the conversion swelling force. It was discussed that the influence factors on limit swelling force and it was indicated by trial that limit swelling force will decrease with increasing of primary water content for remolded soils sample with same dry density. And some representative trial regularity was made clear.