Abstract: In ths paper, the design considerations of the high- efficiency high- gain broadband klystron are described. Its buncher has been studied by using small signal gain and large sigual efficiency programs. The buncher bandwidth of 10% with small signal gain greater than 50 dB can be obtained for 7-cavity klystron. The bandwidth of 10-15% can be obtained for a 8-cavity klystron. The design methods and measured results of two types of output cireuits are introduced. The bandwidth of the filter type output circuit is about 7.5-10% and that of the overlapping mode two gap cavity output circuit is about 10-12%. Two types of S-band broadband klystron have been constructed and tested. Teh experimental results are given as follow. The klystron with a filter type output circuit has 1 dB equi-drive bandwidth of 7.5% with efficiency of 38% and gain of 43 dB. The klystron with an overlapping mode two gap cavity output circuit has 1 dB equi-drive bandwidth of 10%. Their output power is greater than 2.5 MW.