The strength change of alkali-activated slag cement pastes (AASC) exposed to the hot environment from room temperature to 1 000 ℃ was studied. The phase composition and microstructure of AASC were also examined by using DSC/TG, XRD and SEM.The results indicated that the change of AASC activated by water glass was similar to the AASC activated by sodium hydroxide. The loss of evaporative water in AASC at 200 ℃ densified the microstructure of hardened pastes, resulting in an increase in compressive strength but a decrease in flexural strength. When the AASC were heated from 400~ 1 000 ℃, both flexural and compressive strength decreased continually. Particularly, the strength dropped significantly in the range of 600 ~ 800 ℃ due to the phase reaction leading to the formation of gehlenite.