The expansive characteristics of an expansive soils in different concentrations sodium chloride solution were tested by loading-swelling test．The results show that the saturated void ratio decrease as increasing the chloride solution concentration．It is because that the increasing of the sodium chloride solution concentration results the decreasing in the swelling pressure．Based on the results obtained from the samples saturated by distilled water，the relative value of the chemical force produced by different concentration solution is obtained．The relative chemical force increase as the concentration increasing and the void ratio decreasing．And the relationship among the relative chemical force，void ratio and solution concentration is established．