In this paper,the design considerations of a high-efficiency high-gain broadband klystron aredescribed.Its buncher has been studied by using small signal gain and large signal efficiencyprograms.With small gain greater than 50dB,the buncher bandwidths of 10% and 10—15% can beobtained for 7 and 8 cavity klystrons respectively.The design methods and measured results ofoutput circuits of two types are introduced.The bandwidths of the filter type and the overlappingmode two gap cavity output circuits are about 7.5—10% and 10—12% respectively.Two types ofS-band broadband klystrons with the two respective output circuits described are constructed andtested,and the one with the former output circuit has 1dB equi-drive bandwidth of 7.5% with anefficiency of 38% and a gain of 43 dB,while the other with the later output circuit has 1dB equi-drive bandwidth of 10%.Their output power is greater than 2.5 MW. In this paper, the design considerations of a high-efficiency high-gain broadband klystron aredescribed. Its buncher has been studied by using small signal gain and large signal efficiency programs. Small gain greater than 50 dB, the buncher bandwidths of 10% and 10- 15% can beobtained for 7 and 8 cavity klystrons respectively. The design methods and measured results of ofputput circuits of two types are presented. Bandwidths of the filter type and the overlappingmode two gap cavity output circuits are about 7.5-10% and 10-12 % respectively. Two types of S-band broadband klystrons with the two respective output circuits described are constructed andtested, and one with the former output circuit has 1 dB equi-drive bandwidth of 7.5% with anefficiency of 38% and a gain of 43 dB, while the other with the later output circuit has 1dB equi-drive bandwidth of 10%. The output power is greater than 2.5 MW.