Alkali-activated materials are difficult to maintain strength under very high temperatures in the fire. But results are not available for those materials when subjected to sustained moderate temperature range (i.e. 60 °C–250 °C) which is a more common thermal condition in many engineering applications. In this paper, the mechanical and microscopic properties of alkali-activated fly ash-slag (AAFS) materials under sustained moderate temperature effects are studied. The microscopic properties are characterized by XRD, FT-IR, 29Si NMR-MAS, MIP and SEM-EDS. The results show that AAFS is able to maintain the compressive strength even being treated under 250 °C for 28d. The microscopic characterizations indicate that N-A-S-H gel exhibits greater thermal stability than C-A-S-H and C-S-H gels. However, after the gel loses water under sustained temperature effect, N-A-S-H is hard to reorganize. In other words, when more N-A-S-H is produced due to the addition of more slag, the AAFS becomes more porous with looser microscopic morphology leading to lower compressive strength. It is also found that, after sustained moderate temperature treatment, the gel reorganizes and agglomerates into cluster-like particles, while the matrix becomes more uniform and compact. The sustained temperature helps refine the pore size and improve the volume of connected pores and super-macropores which is helpful to maintain the compressive strength of AASF. This study provides experimental and theoretical basis for the application of alkali-activated materials subjected to sustained moderate temperature effects.