Abstract: Sulfate attack-induced expansion of cement-treated aggregates in seasonally frozen regions is a well-known issue which causes continuous expansion in road and railway subgrades, and particularly in high-speed railways. Accordingly, we investigate the influence of material proportions, the number of freeze-thaw (FT) cycles, and temperature gradients on the expansion mechanism of sulfate attack on cement-treated aggregates subjected to FT cycles. The conditions, laws, and dominant factors causing expansion of aggregates were analyzed through swelling tests. The results indicate that under FT cycles, 3% content cement-treated graded macadam only experiences slight deformation. The maximum strain of graded macadam attacked by 1% sodium sulfate content in each FT cycle is significantly larger than that of 3% content cement-treated graded macadam attacked by 1% sodium sulfate content. Using scanning electron microscopy, needle-like crystals were observed during sulfate attack of cement-treated graded macadam. Through quantitative analysis, we determined the recoverable and unrecoverable deformation of graded macadam under FT cycles. For graded macadam under sulfate attack, the expansion is mainly induced by periodic frost heave and salt expansion, as well as salt migration. For cement-treated graded macadam under sulfate attack, the expansion is mainly induced by chemical attack and salt migration. This study can serve as a reference for future research on the mechanics of sulfate attack on cement-treated aggregates that experience FT cycles, and provide theoretical support for methods that remediate the expansion induced by sulfate attack.