Abstract: In order to realize the long distance transmission of discharge initiated repetitive-pulsed HF laser, and produce a high quality laser of large volume in the shorter resonant cavity, the structural design, simulation calculation and experimental study of the passive branch confocal unstable resonant cavity were carried out. The simulation results show that with the increase of the magnification M, the energy of far-field light focal spot was gradually increased with the increase of magnification, and the energy transferred to the light spot center. In addition, the light spot size and the divergence angle of far-field were gradually decreased as well. The experimental results show that with the increase of the magnification M, the variation of the far-field intensity distribution, the light spot size and the divergence angle were consistent with the simulation results. But the output laser energy actually increases first and then decreases. Considering the requirements of high beam quality and high energy, in the case of normal flow field, the far-field divergence angle of repetitive-pulsed HF laser was 2.37 times diffraction limit magnification when M was 3, and the laser energy was slightly lower than the stable resonant cavity(about 94.6% of stable resonant cavity), which satisfies the requirements of long distance transmission.