The fiber-reinforced metakaolin-fly ash (MK-FA) based geopolymer mortar was prepared from the main raw materials of metakaolin and fly ash. The appearance, microstructure, mass loss rate and mechanical properties of fiber-reinforced MK-FA based geopolymer mortar were explored after sulfate corrosion and drying-wetting alternation coupling, and the improvement mechanism of sulfate resistance of mortar by fibers was analyzed. The results show that with the increase of sulfate corrosion and drying-wetting alternation cycle times, the mechanical performance of specimens first increase and then tend to be stable, while the mass loss first increases and then decreases, fibers improve sulfate resistance of geopolymer mortar. The sodium sulfate solution mainly erodes the surface of the specimens, does not generate gypsum and ettringite, and has no obvious damage to the internal structure. Both stable and dense structures of samples and fiber-bridged matrix function make fiber-reinforced MK-FA based geopolymer mortar present excellent performance in resistance to sulfate attack，which is beneficial to maintain the mechanical properties and structural stability of specimens.