Abstract: The expansive soil is classified as the unsaturated one in engineering construction. The swelling and shrinkage deformation of the expansive soil may cause intensified line harshness for the ballastless track on high-speed railway subgrade, affecting the normal operation of high-speed railway. To study the deformation characteristics of unsaturated expansive soil layer under lining of railway subgrade after artificial soaking, considering the construction of Kunming-Nanning Railway, field soaking tests on expansive soil foundation under railway subgrade are designed and carried out. Meanwhile, the time histories of deformations of expansive soil foundation and subgrade body, and moisture migration of shallow clay are measured from the beginning of subgrade filling to the end of artificial soaking. The test results reveal the ultimate deformation of relative expansion of expansive soil foundation after being soaked and saturated, distribution of swelling deformation along the cross section of subgrade and the depth of foundation as well as decay characteristics of swelling deformation of ground surface along the subgrade body. Based on the test results, the objective of the subgrade surface without swelling deformation control, which is used to determine the critical height of subgrade, is initially derived. The design principles and implementation methods of field tests can be used as a reference method for studying the swelling and shrinkage characteristics of expansive soil foundation under railway subgrade.