Abstract: Free swelling ratio is one of the most important indicators for the expensive soils. The laboratory test for the free swelling ratio needs a long period and has many influencing factors for the outcome. Besides, the test reproducibility is poor. The free swelling ratio and soil conductivity are both closely related to the soil cation exchange capacity and specific surface area. But when the soil moisture content is under certain conditions, the soil conductivity is a relative stability indicator. Therefore, to use the electrical conductivity to measure the free swelling rate of the expansive soil can solve the difficulties in tests for the free swelling rate. Here, certain moisture content is adopted to test the electrical conductivity and free swelling ratio of the expansive soil samples. The test results show that the soil conductivity grades can be used to reflect the expansion level of the expansive soil. Under certain moisture content, their free swelling ratio can be determined by measuring the electrical conductivity of the expansive soil samples. These achievements are of realistic significance for the researches on applying electrical conductivity to the classification of the swelling and shrinking grades of the expansive soils.