Abstract: A 4.6% instantaneous bandwidth 17 kW CW S-band klystron KS4158 for use in communication system is developed,its broadband output circuit is investigated specially, and two types of circuits, which are filter loaded double-gap coupling cavity output circuit with mode coalescence and three-pole filter loaded single cavity output circuit, are designed in this paper. A new method of measurement of broadband output circuit using vector network analyzer by frequency sweep, and the measured results of the two types of circuits are presented in this paper. And also the test results of the klystrons with two types of broadband output circuits are presented and discussed. As result, the klystron with three-pole filter loaded single cavity output circuit gave >17 kW CW output power and >4.6% instantaneous bandwidth.