In order to investigate the strength development rules and the improvement mechanism of granulated blast-furnace slag (BFS) modified fly ash based geopolymer mortar at different temperatures, the mechanical properties of fly ash based geopolymer with different BFS content at various temperatures were measured, and the microscopic morphology and microscopic pore structure characteristics were analyzed. The results show that the compressive strength and flexural strength of fly ash based geopolymer are small when curing at room temperature, and the mechanical properties of fly ash based geopolymer can be improved by mixing BFS or elevating curing temperature. However, high temperature curing probably lead to a slow development of later compressive strength. When not mixed with BFS, the flow degree of the geopolymer is 232mm, and the condensation time exceeds 8 h. With the increase of BFS content, the flow degree and condensation time of the geopolymer decrease gradually. Both elevating curing temperature and mixing BFS can significantly reduce the porosity of fly ash based geopolymer. When just cured at room temperature, the geopolymer mortar contains a large amount of macroscopic pores, and there is substantially no gelling pores. Through high temperature curing, the proportion of capillary pores becomes the largest in the fly ash based geopolymer mortar, and the modified geopolymer is mostly composed of gel pores and transition pores. It can be seen from the microtopography of the specimens that geopolymer becomes more dense after mixed with the BFS; the fractal model based on thermodynamic relation can perfectly describe the pore structure profile of the geopolymer within the measurement range of mercury intrusion porosimetry, followed by the pore axis model; the fractal dimension of pore structure of the specimens is greater than 2． 0. Adding BFS to the fly ash based geopolymer can improve the pore structure, while lifting curing temperature makes the pore structure of the geopolymer complex.