Salinized subgrade fill soil in cold regions typically comprises composite saline soil rich in sodium chloride and sodium sulfate.This study aimed to explore the mechanism of the effect of chloride ion content on salt expansion of the salinized subgrade in cold regions.Based on the calculation theory of the sodium chloride-sodium sulfate-water ternary system solution in the Pitzer ion model and combined with the FREZCHEM model，the study quantitatively analyzed the intrinsic connection between water activity，freezing factor，and supersaturation ratio with temperature and concentration ratio （η=mCl-/mSO2-4）.It investigated the variation patterns of the freezing temperature and crystallization supersaturation ratio of composite saline soil with sodium sulfate content and concentration ratio，revealed the impact mechanism of sodium chloride on the crystallization of sulfate saline soil，and determined the critical concentration ratio for adding sodium chloride to sulfate saline soil to reduce salt expansion.The results show that the FREZCHEM model can be used to analyze composite saline soil ’s freezing and salt crystallization characteristics.The ice formation factor is closely related to temperature and solution concentration，and when it equals 1，the pore solution is in thermodynamic equilibrium.Sodium chloride has a dual role in promoting or inhibiting the crystallization of sodium sulfate.When the concentration of sodium sulfate in the saline soil exceeds 1.0 mol/kg，the salt expansion deformation of sulfate saline soil will be significantly reduced by adding sodium chloride according to a critical concentration ratio greater than 2.5.Meanwhile，the salinization method applied during on-site construction can effectively suppress the salt expansion and deformation of the roadbed in saline cold areas.