Abstract: As a critical component of fiber-reinforced cement-based composite materials, the interface plays a pivotal role in determining the mechanical behavior of such composites. This study focuses on sisal fiber-reinforced geopolymer composite (SFRGC), modified sisal fibers using different methods such as alkali treatment, acid-alkali treatment, and nano-SiO2 adsorption to improve interface compatibility. To investigate the interface toughening mechanism of SFRGC under different modification processes, the compressive strength and flexural strength of SFRGC were studied. SEM、FTIR and EDS were used to analyze the microstructure and physical properties of the interface. And digital image correlation (DIC) was utilized to explore the crack propagation and strain evolution patterns of SFRGC. The results showed that when sisal fibers were pretreated with acid-base treatment and the nano-SiO₂ concentration was 2%, the compressive strength and flexural strength of the composite materials increased by 18.8% and 66.0%. Based on the analysis of the strain evolution process throughout the entire field, modified sisal fibers significantly improved the residual strength and ductility of the specimens by complicating the crack propagation path. The introduction of nano-SiO₂ promotes the condensation and hydration reactions of geopolymers, enhances chemical adhesion, and combines mechanical interlocking and frictional resistance to synergistically inhibit fiber pull-out, thereby optimizing strain hardening behavior and improving interface bond strength.