Abstract: As a low-carbon cementitious material, the application of fly ash based geopolymer in ground improvement has attracted more and more attention. However, the engineering characteristics of the soil stabilized by alkali activated cementitious material under the freezing-thawing extreme climate conditions are not clear. It is necessary to further study the strength, deformation characteristics and their influencing factors of the improved soil under the freezing-thawing cycle. Several laboratory tests are carried out to investigate the effects of the ratio of silicon to aluminum in raw material (Si/Al), modulus of alkali-activator and alkali solution concentration on the unconfined compressive strength (UCS) and the freezing-thawing resistance of fly ash based geopolymer stabilized soil, and the corresponding micro mechanism. The results show that (1) the UCS of geopolymer stabilized soil decreases with the increasing alkali-activator modulus and the decreasing alkali solution concentration, while increases with the Si/Al value in the range of 1.15 to 1.35, and the unconfined compressive strength of 28 d geopolymer stabilized soil can reach 8.98 MPa. (2) When the Si/Al value changes from 1.25 to 1.35 and the alkali solution concentration is within the range of 5.42 to 22.78 mol/L, the 28 d geopolymer stabilized soil can resist more than one (up to 6) freezing-thawing cycle. (3) The best performance of the freezing-thawing resistance of fly ash based geopolymer stabilized soil is mainly related to large number of Si-rich aluminosilicate gel generated by polymerization, while the highest compressive strength is related to the amount of Al-rich aluminosilicate gel generated by polymerization. The research results will provide technical reference for the mix design of soil stabilization with fly ash based geopolymer, and promote the application of alkali activated cementitious material in ground improvement.