expansive soil;moisture content;dry density;swelling pressureIn order to further study of the law development of expansive soil expansion force, load balance method is used to study the Nanjing expansive soil vertical expansion force changing with time, initial moisture content, and dry density. The results show that the smaller the initial moisture content of expansive soil sample, the bigger the dry density is, the faster the expansion rate is;Expansion force decreases with the increase of initial water absorption, The expansion force and water content of expansive soil samples are linear. The expansive force increases with the increase of dry density, and there is exponential function between expansion force and dry density of expansive soil sample.