In order to explore the polymerization mechanism of granite residual soil, geopolymer containing fly ash, various particle groups, granite residual soil and their composites were prepared with different materials. Sodium hydroxide and sodium silicate are used as alkali activators. In this paper, the compressive strength of geopolymer was studied. The mineral composition and microstructure were tested and analyzed by X-ray diffraction and scanning electron microscope. The results show that geopolymerization can only occur in fine particles due to the presence of amorphous aluminosilicate in granite residual soil. The zeolite phase transition of low polymer in fine-grained reaction is beneficial to enhance the integrity of the sample and improve its compressive strength. The addition of fly ash can accelerate the geopolymerization rate and improve the strength of fine geopolymer, but it can inhibit the occurrence of zeolite phase transformation.