In order to test the uniformity of the dielectric properties of fiber monofilaments, this paper adopts a ridged semi-cylindrical cavity to test the dielectric properties of fiber monofilaments, which can provide a highly sensitive testing environment through electric field compression. Combined with the resonant cavity perturbation method, the permittivity and loss angle tangent of the fiber monofilament are calculated. A test system was set up using a vector network analyzer, a ridged semi-cylindrical cavity, a fiber fixing support, a three-dimensional moving platform, an acrylic isolator, a fiber fixing support, an optical platform, and test software, and the permittivity and loss angle tangent of fiber monofilaments were tested and analyzed. The results show that for fiber monofilaments with a diameter of twenty or so microns, sufficient test sensitivity is available. By testing quartz fibers, glass fibers and silicon nitride fibers and comparing them with reference values that can be consulted, the differences are small. The test system is suitable for testing the dielectric properties of fibers and their uniformity.