<p>Treating sulfate saline soil with lime or cement can lead to significant ettringite-induced swelling, resulting in the deterioration of subgrade or foundation layers, particularly in the presence of excessive moisture. This study employed reactive MgO and ground granulated blast furnace slag (GGBS) to treat saline loess to mitigate this adverse swelling. Sodium sulfate saline loess with 4% salt content was stabilized by 8%, 10%, and 12% MgO-GGBS binder with MgO to GGBS ratios of 1:9, 2:8, and 3:7. The properties of the treated soil was assessed through linear swelling test, unconfined compressive strength (UCS), X-ray diffraction (XRD), derivative thermogravimetric analysis (DTG), and scanning electron microscopy (SEM) analysis. The results demonstrated that the UCS of saline loess stabilized with the MgO-GGBS binder increased gradually with the GGBS dosage and the curing time, and the MgO content of the binder exceeded 1% favored the hydration of GGBS and the strength development of cured saline soil. In addition, the linear swelling (%) and strength loss (%) of saline soil stabilized by the MgO-GGBS binder under immersion conditions decreased progressively with increasing MgO dosage. Saline loess cured with the MgO-GGBS binder containing 2% MgO exhibited the lowest linear swelling of 0.08%, while the strength of the cured soil with MgO dosages greater than 2% gradually increased under immersion. The optimal formulation for stabilizing saline loess with 4% salt content was 10% MgO-GGBS binder containing 2% MgO. XRD, DTG, and SEM analysis confirmed that the major secondary reaction products formed in the cured saline soil matrix by the MgO-GGBS binder containing more than 2% MgO included low crystallinity ettringite, S-AFm, Mg–Al–SO₄ LDHs, and MSH gel, which primarily contributed to the reduction in linear swelling of the cured soil. In contrast, the swelling of binder treated soil with less than 1.6% MgO was predominantly due to the formation of ettringite and its water-absorption expansion.</p>