Abstract: To address the brittleness of one-part geopolymer(OPG)stabilized soft soil,this study used polyester(PET)fiber as a toughening agent. The effects of PET fiber content,fly ash content,and curing age on the unconfined compressive strength,splitting tensile strength,post peak stress strain behavior,and failure mode of OPG-stabilized soft soil were investigated. Based on the experimental results,a response surface methodology was applied to develop a predictive model for the splitting tensile strength of PET-fiber-reinforced OPG-stabilized soft soil,incorporating multiple parameters. The results indicate that PET fiber has little effect on the unconfined compressive strength of OPG-stabilized soft soil,but splitting tensile strength increases with higher fiber content. The post peak stress strain curves show a more gradual decline with increasing PET fiber content,reflecting improved material toughness. Crack development in the stabilized soil is also reduced with higher fiber content. The addition of fly ash significantly lowers both unconfined compressive strength and splitting tensile strength,yet it enhances the overall toughness of the material. Furthermore,the toughening effect of PET fiber is more pronounced in OPG-stabilized soil containing fly ash. The developed predictive model accurately describes the splitting tensile strength of OPG-stabilized soft soil with varying fiber contents. These findings offer new insights into the application of PET fiber in OPG-stabilized soft soils.