Abstract: A high-speed railway subgrade which experiences a continuous and severe heave after construction is investigated through the delamination deformation monitoring, expansive tests and XRD tests. Sulfate attacking on cement-stabilized filler is the main reason of subgrade heave, and the reaction conditions and the development process of sulfate attacking expansion are analyzed based on the mechanism of crystal growth and chemical reaction. The results show that the cement-stabilized filler where the ettringite and thaumasite are identified has obvious swelling deformation, and the subgrade heave relates to the crystal formation of ettringite and thaumasite caused by sulfate attacking on cement-stabilized filler. The relatively humid alkaline environment and existence of sulfate minerals in cement stabilized filler are the necessary condition for such sulfate attacking expansion. The sulfate attacking on cement-stabilized filler exhibits a long-term persistent heave characteristic, and typical expansion of this type may develop for several years to create a critical and dangerous situation.