Abstract: In order to study the working performance and mechanical properties of geopolymer mortar prepared by different types of fly ash, the microstructure characteristics of geopolymer mortar were systematically studied by pore structure determination, Fourier transform infrared spectroscopy and scanning electron microscope-energy spectrum analysis. The results show that the fluidity of geopolymer mortar prepared with grade Ⅰ fly ash is slightly lower than that of geopolymer mortar prepared with grade Ⅱ fly ash, which is mainly related to the physical properties of fly ash, and the influence of fly ash average particle size on geopolymer mortar performance is greater than that of water demand ratio. The fluidity of geopolymer mortar prepared with coarse fly ash is relatively poor. The compressive and flexural strength of geopolymer mortar prepared with grade Ⅰ fly ash are better than those prepared with grade Ⅱ fly ash. The chemical composition of fly ash, especially the ratio of SiO₂ to Al₂O₃, has the greatest influence on the mechanical properties of fly ash-based geopolymer mortar. The fly ash containing higher SiO₂/Al₂O₃ can improve the compactness of the matrix, which is characterized by a large number of evenly distributed fine pores. In engineering practice, it is suggested to give priority to the selection of fly ash with suitable particle size and high SiO₂/Al₂O₃ ratio, which has optimized the performance of geopolymer materials.