The time-effect characteristics of shear displacement of soil under different shear stress levels are analyzed. Four states of soil deformation are identified as the shear stress level increases, namely, fast steady, slowly steady, slowly breaking and fast breaking states. Based on the creep direct shear test of silty clay with different degrees of compaction, these four states are investigated. The test data are fitted by a power function which serves as the classification criterion of deformation state. Based on the shapes of shear stress level-power curve, three shear stress level thresholds corresponding to these four states are obtained. An expression between the state strength parameter and the ultimate strength parameter is developed based on the Coulomb strength equation. The post-construction settlement of subgrade in high-speed railway is of a magnitude of millimetre. It is found that the choice of embankment fill material and the compaction standard must meet the control principle and technology condition of fast steady state. The research results provide theoretical and experimental bases for controlling deformation states of soil structures in high-speed railway subgrade.