Vertical cutoff wall is one of the most effective technologies to restrict the migration of contaminants. In this study, a geopolymer cutoff wall backfill (GCWB) consisting of reactive sodium silicate, fly ash, sand, and water was developed, which does not need Ordinary Portland Cement (OPC) and bentonite. The basic macroscopic properties of GCWB were tested, in terms of workability, unconfined compressive strength, hydraulic conductivity, and chemical compatibility, then the optimal material ratio was determined. In order to evaluate the barrier performance of GCWB under dry-wet cycles, the hydration products and microstructural characteristics were measured by X-ray Diffraction (XRD), Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). According to the orthogonal analysis, the optimal material ratio with the highest unconfined compressive strength was 30FA1.2M12S and that with the lowest hydraulic conductivity was 40FA1.2M12S. The unconfined compressive strength and hydraulic conductivity of GCWB can achieve 6.62 MPa and 4.83 × 10 m/s after cured for 28 days, respectively. The chemical compatibility of GCWB was overall favorable in NaSO and CaCl solutions. Under the effect of dry-wet cycles, the variation of hydraulic conductivity, mass and volume in contaminant solutions were basically the same as that in water. The hydraulic conductivity increased sharply by two orders of magnitude after the 1st dry-wet cycle and tended to be stable (slightly larger than 1 × 10 m/s) in the subsequent dry-wet cycles. Therefore, the barrier performance and durability of GCWB was desirable in water and contaminant solutions, indicating that it is a promising antifouling cutoff wall material.