Abstract: In response to the demands for high-end measurement instruments and optical communication testing, a ultra-wideband single-pole double-throw coaxial switch with a working frequency range of DC~90 GHz has been designed. By optimizing the overall structure of the switch and using HFSS software to simulate and optimize the matching of the coaxial connector, shielded stripline, and the transition section between coaxial and shielded stripline, combined with overall matching optimization, and ultimately achieve ultra-wideband matching transmission. The final design was physically tested on a manufacture d DC~90 GHz ultra-wideband coaxial switch, verifying the correctness of the design method and the accuracy of the design results. The results show that within the working frequency range of DC~90 GHz, the voltage standing wave ratio of the coaxial switch is less than 1.65, the insertion loss is less than 1.20 dB, the isolation is greater than 70 dB, and the load end standing wave ratio is less than 1.60, which can meet various high-end testing needs such as microwave and millimeter-wave testing and optical communication testing.