Geopolymers,such as phosphate-based geopolymer,are promising materials that have drawn extensive attentions as wave-transparent candidates with low dielectric loss factor.In this paper,multilayer SiO2 fibers are introduced into phosphate-based geopolymer matrix to prepare composites.Crack generation and fracture behaviors during the three-point bending tests in as-prepared SiO2f/phosphate geopolymer composites are clarified by in situ observation.Before high-temperature treatment,composite with 17 vol％ fiber content exhibits the best mechanical properties.A decreasing mechanical strength is observed after being treated at higher temperature,which is attributed to the stronger interface binging between fibers and phosphate-based geopolymer matrix.Most importantly,the presence of SiO2 fibers can strengthen mechanical properties of phosphate-based geopolymer matrix but shows negligible impact on its dielectric properties,achieving dielectric loss factor as low as 3.53 × 10-3 after being treated at 700 ℃.