Free swelling ratio is an important index for identification and classification of expansive soil. In the process of its tests many artificial factors exist，and its scientific and reliability are widely doubted，so searching an adequate and applicable index with fewer artificial factors and representing the nature of expansive soil is of significance. A new kind of testing measure is introduced. That is relative free swelling ratio test. The relative free swelling ratio is defined as the ratio of the equilibrium sediment volume of 10 g oven-dried soil in distilled water to that in coal soil(or CCl4). The significance of the index and the testing measure are described. Relative free swelling ratio and physical and mechanical properties tests for expansive soil are conducted systematically. From the tests it is found out that when compared to free swelling ratio，relative free swelling ratio shows better correlation with cation exchange capacity，specific surface and montmorillonite content，so it is concluded that relative free swelling ratio is consistent with the index which exhibits the nature of expansive soil. The credence is compared with the indices suggested by specifications for highways including liquid limit，plastic index and standard moisture absorption water content and the results indicate that the former shows a higher credence and possesses more feasibility，meanwhile the correlations between the relative free swelling ratio and other indices are good，so these indices should be considered together for identification and classification of expansive soil.