Abstract: The free swelling ratio of modified Ankang expansive soil with KCl solution at different concentrations was tested;the effect of KCl solution with different mass fractions on the expansive soil was analyzed;and the swelling mechanism of Ankang expansive soil was further discussed.The free swelling ratio test of the modified expansive soil was conducted with different mass fractions of KCl solution,being 3%,6%,9%and 12%,respectively.The results show that comparing with the plain expansive soil,the KCl solution would reduce the free swelling ratio of theexpansive soil.With theincrease of KCl concentration,the free swelling ratio of the soil samples decreased at first,then increased and finally decreased again,showing an overall fluctuant decreasing tendency.Influenced by KCl solution with concentration of 3%,6%,and 9%,the free swelling ratio of expansive soil decreased and became weak expansion potential soil.The free swelling ratio of soil samples reached the lowest value being 33%due to theimpact of 12%KCl solution,and the expansive soil became non-expansive soil.According to the results,and focusing on the effect of the crystal layer expansion and its inhibition,as well as the concentration change of the salt solution between the pores on the expansion characteristics of the expansive soil,four effects on the fluctuation of the swelling ratio were studied,and the primary and secondary relationships of various effects in different concentration intervals were analyzed emphatically.Based on the theory of crystal layer expansion and the theory of permeability,the expression of the expansion potential of expansive soil was proposed.The partial coefficient which affects the expan-sion potential was discussed in details,showing that the effects of each partial coefficient in different concentration intervals are different.This expression could reasonably explain the change of the free swelling ratio of the expansive soil,which firstly decreased,then increased and finally decreased.It is determined that the fluctuant tendency is caused by the relative strength of the crystal layer expansion and its inhibition,as well as the concentration change of the salt solution between the pores.This theory is expected to make up the deficiency of the theory of the crystal layer expansion.