Some fine aggregates were substituted for glass sand in the preparation of alkali-activated slag (AAS) mortars. The effects of glass sand content (0%, 10%, 20%, 30%, mass fraction) on compressive strength, flexural strength, drying shrinkage, thermal conductivity, and expansion rate of alkali-silicate reaction (ASR) of AAS mortars were tested, and the microscopic mechanism was analyzed by scanning electron microscope (SEM). The results demonstrate that the early compressive strength of AAS mortars can be significantly improved with 10% to 30% glass sand content, but the 28 d compressive strength can be slightly reduced. The flexural strength of AAS mortars increases first and then decreases with the increase of glass sand content.The 10% glass sand content is most favorable to the 3 d flexural strength, while the 20% glass sand content is most favorable to the 28 d flexural strength. With the increase of glass sand content, the drying shrinkage, thermal conductivity and ASR expansion rate of AAS mortars all decrease. The AAS mortar with 30% glass sand content is discovered that it has 14.4% lower thermal conductivity, 27.6% lower 56 d drying shrinkage, 39.6% lower 14 d ASR expansion rate, and 34.5% lower 28 d ASR expansion rate comparing to the control group. Hydration products are created on glass sand surface, and its binding with cementitious material is tighter than that of quartz sand, according to SEM examination, which makes the microstructure of AAS mortar compact.