Abstract: The subgrade defect of expansive soil cutting is one of difficult problems in railway construction. Based on the analysis results of the character and mechanism of subgrade defects, semi-rigid water proof structural layer is studied, and fully-enclosed subgrade is designed. In order to study the dynamic character of this new type of railway subgrade, a 100 m-long fully-enclosed subgrade is constructed in typical expansive soil cutting section of Kunming-Nanjing high-speed railway, and in-situ dynamic tests are carried out. The results show that: (1) The dynamic stress, velocity and acceleration are affected by the working conditions. Compared to those under the dry conditions, all the three dynamic indexes become larger when the surface of subgrade is soaked. (2) The dynamic displacement of subgrade surface decreases with the increasing distance, and the attenuation curve can be fitted by power function: y=0.562x^(-0.37). (3) The relation curve between dynamic stress and excitation frequency has double peak phenomena, and the peak frequencies are 10 and 18 Hz, respectively. () The excitation frequency spectrum is 16～21 Hz when the velocity and acceleration increase rapidly.