The potential application for fire resistance requires the foam geopolymers being in no-cracking state and good thermostability during heat exposure.Fly ash based foam geopolymers reinforced with polyacrylonitrile (PAN) fiber were synthesized in this study, aiming to achieve its brittleness modification and evaluate the thermal evolution of geopolymer matrix with the incorporated fibers at different calcination temperatures.The results showed that the incorporated PAN fiber (0.1 wt.%) could significantly improve the mechanical strengths and slightly lower the bulk density.High temperatures (>600 ℃) exposure have been shown to lead to significant increase in mechanical strengths, which has been evidenced that was mainly due to the fusion of geopolymer gel and crystals formation.In addition, during high temperatures exposure, the SEM investigation indicated complex thermal evolution reaction between geopolymer gels and PAN fibers.The buried PAN fibers could still be observed after 800 ℃ calcination, revealing that the geopolymer matrix provided a protective barrier against the degradation of the fibers.