The present study fabricated polyvinyl alcohol (PVA) fiber-reinforced alkali-activated slag/fly ash (AASF) composites with varying mixture ratios of slag and fly ash. The thermomechanical behaviors of the AASF composites exposed to 200, 400, 600, or 800oC were evaluated by means of compressive strength test, visual observation, and fire resistance tests. Xray diffractometry, mercury intrusion porosimetry, and thermogravimetry tests were performed to analyze the microstructure change of the AASF composites upon exposure to high temperatures. Specimens exhibited a gradual strength loss up to 600oC, while also showing a significant decrease in the strength above 600oC. The fire resistance test revealed the occurrence of an inflection point as indicated by an increase in the internal temperature at around 200oC. In addition, specimens showed the dehydration of C-S-H gel, the presence of akermanite, gehlenite, and anorthite upon exposure to 800oC, which is associated with the formation of macropore population with pores having diameters of 1-3 um and 20-40 um. Visual observation indicated that the PVA fibers mitigated the cracking and/or spalling of the specimens upon exposure to 800oC.