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Sulfate attack is one of the non-negligible factors that induces deterioration in the performance and life cycle of soil stabilizers. In this paper, the degradation mechanism of the durability of slag–fly-ash-based geopolymer stabilized soft soil (hitherto referred to as SF-GSSS) under the sodium sulfate (Na₂SO₄) and magnesium sulfate (MgSO₄) attack environment is comparatively investigated, and the slag/fly ash ratios are set to S10F0, S9F1, S8F2, and S7F3. The SF-GSSS was fully immersed in a 2.5% Na₂SO₄ solution and 2.5% MgSO₄ solution, respectively, to characterize the deterioration rules via visual observations, an unconfined compressive strength (UCS) test, and by mass change. The effect of sulfate on the microstructural characteristics of the SF-GSSS were determined by different microanalytical means, such as by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the SF-GSSS immersed in a MgSO₄ solution displayed significant physical deterioration, but not when in a Na₂SO₄ solution. The mass growth of the SF-GSSS when immersed in a Na₂SO₄ solution was significantly lower than when it was immersed in a MgSO₄ solution at the same immersion age. The rate of strength loss was lowest for S9F1 and highest for S7F3 at the end of immersion, regardless of its immersion in Na₂SO₄ or MgSO₄ solutions.