The invention discloses a fly ash based geopolymer grouting material including, by weight, 20-40 parts of an alkali activator, 30-50 parts of fly ash, 0-10 parts of metakaolin, 0-10 parts of cement, 60-90 parts of an aggregate, 0.05-0.1 parts of a retarder, 0.03-0.05 parts of a defoamer, 0.01-0.03 parts of a water-retention thickener, and 10-30 parts of water. The invention also provides a preparation method for the geopolymer grouting material. The preparation method is simple. The geopolymer grouting material has excellent flowability and workability, is micro-expansive, has high early-stage and later-stage strength, is free of water bleeding, is easy to construct and has good durability, can replace a conventional cement-based grouting material for fixing of foundation bolts and secondary grouting of equipment base or steel-structure column base floor, and effectively solves the problems of low flowability, short setting time, poor workability, low later-stage strength, high cost and shrinkage and cracking during usage of the grouting material. In the invention, industrial waste resources, such as the fly ash, are reasonably and effectively utilized to achieve environment protection and energy saving.