Abstract: We addressed the problem of relative narrow bandwidth of S-band klystron,originated from adoption of resonant cavity as its high frequency interation circuit.The solution to broaden the output band was the self-developed output circuit of double-gap overlapping-mode coupling cavity,comprising the 1^st-cavity,2^nd-cavity and coupling slot.First,the design of novel double-gap overlapping-mode output circuit was optimized via theoretical analysis and numerical simulation; next,the output circuit was approximated with an equivalent circuit in calculation of the impedance-frequency characteristics,and the prototyped novel output circuit was cold tested with satisfactory results; and finally,the S-band klystron integrated with the output circuit was experimentally evaluated via hot test.The hot-test results show that the peak output power was greater than 800 kW in 250 MHz range,satisfying the stringent design requirements.