Sulfate attack is an important factor affecting the durability of concrete. In order to further improve the sulfate attack resistance of high belite sulfate-aluminate cement (HB-CSA) based concrete, the granulated blast-furnace slag (GBFS) was added into HB-CSA to improve the sulfate attack resistance of HB-CSA. Corrosion resistance coefficient was used to evaluate the effect of slag on the sulfate attack resistance of HB-CSA mortar. Combined with X-ray diffraction analysis, thermogravimetry, mercury injection, scanning electron microscope and other testing methods, the mechanism of sulfate attack resistance was studied from the microscopic level. The results show that slag can improve the sulfate attack resistance of HB-CSA, and its corrosion resistance coefficient increases to 1.51. In the process of GBFS-HB-CSA resistance to sulfate attack, slag particles play a major role in the filling and refining of the cement pore structure of HB-CSA. SO2-4 and Ca(OH)2 jointly stimulate the hydration of Al2O3 and SiO2 in the slag to produce gel and ettringite (AFt). The gel and AFt fill in the slurry pores to refine the pore size of the slurry, which makes the slurry structure dense and reduces the ionic permeability, thus improving the ability of HB-CSA to resist sulfate attack.