Abstract: In order to improve the engineering properties of undisturbed subgrade soil, systematic indoor experiments were made to study the expansive soil for the railway subgrade filling in Xi'an-Nanjing railway subgrade project. Physical and mechanical parameters of the expansive soil were measured by the basic tests of geotechnical properties, such as the direct shear test and tri-axial test. According to the actual engineering properties of the expansive soil, a soil improvement measure using lime was put forward. Parameters such as the particle size distribution, physical properties, water-physical properties, strength, and expansibility of the expansive soil were tested and compared before and after improvement. The test results show that the weak-middle expansive soil gets an obvious improvement in particle composition, physical properties, and swelling behavior; and the mechanical strength and water stability are also enhanced greatly. According to the test results of mechanical and physical properties, the optimal ash mixing rate for the expansive soil is determined to about 5%.