In this paper, cyclic triaxial loading tests were conducted on specimens of soft clay with varying cyclic stresses and frequencies to investigate the performance of soft soil subgrade subjected to cyclic loading. The laboratory results indicate that given the same level of cyclic stress, the stability of clay subgrade is primarily dependent on the loading time with no consistent frequency/train speed effect. There exists a critical level of cyclic stress between 60 and 80% of the monotonic shear strength, above which failure may occur regardless of the loading frequency. The nature that soils behave dependently on cyclic stress level rather than loading frequency was investigated through the strain rate during cyclic loading, which is considered responsible for the cyclic response of soft clays under various loading conditions. For loading frequencies ranging from 0.1 to 5 Hz, it was found that the strain rate depended on the cyclic stress ratio rather than the loading frequency, which implies that the cyclic stress level plays a more important role in influencing the cyclic performance of soft soil subgrade.