A 5.5-m diameter frame radome was designed and fabricated in order to meet the requirements of 100 MHz–20 GHz frequency, high average transmission power coefficient and harsh environment of strong wind and severe cold in Ali Primary Gravitational Wave Observatory. The frame is made of high-strength basalt fiber composite material and the skin is made of PTFE coating material with lower dielectric constant and loss tangent, which can greatly reduce the electromagnetic transmission loss. According to the semi-positive 32-plane expansion block division method, the whole is divided into 80 triangular unit plates for splicing to minimize electromagnetic barrier and electromagnetic scattering. The basalt fiber frame radome does not cause secondary radiation problems by the induced current rate of metal frame, and the main factors affecting the insertion loss are the loss of PTFE film and the blocking effect of frame rib array. The antenna pattern with and without radome is compared and analyzed by simulation software. It shows that the radome has little influence on the main lobe of the antenna pattern. The maximum directional loss in the antenna main lobe does not exceed 0.02 dB. The test shows that the maximum absolute insertion loss of the radome at 18 GHz frequency is 0.0175 dB, which is close to the simulation values.