Abstract: A broadband coaxial transmission line holder testing device was developed for existing testing equipment cannot meet the requirement of SE broadband test of shielding materials against electromagnetic pulse in timedomain.Firstly，the dimension of coaxial inner and outer conductors and the transition section were analyzed. Then， the support medium of coaxial transmission line holder was optimized by using the particle swarm optimization algorithm combined with the electromagnetic field numerical analysis software.Measurement results show that the return loss of the developed coaxial transmission line holder device is less than -20 dB below the 64 GHz.The broadband coaxial transmission line holder not only extends the highfrequency limitation to 64 GHz，but also ensures the lowfrequency up to DC.Testing materials sample can be smaller，and the test curve is smoother.