Abstract: This paper presents a new type of coaxial cavity loaded with a ridged cavity and a filter as the output circuit of S-band multiple-beam klystron. Theoretic analysis and equivalent circuit of the output circuit are given based on the theory of coupling resonance cavity. The uniformity of the gap field distribution in the output cavity and the gap impedance varying with frequency of output circuit are studied and simulated by means of CST-MWS simulation software. The results show that coaxial output cavity of the output circuit with coaxial compound cavity loaded with a filter not only reduces the request for the high current density emitted by the cathode, but also realizes 128 MHz/dB broad bandwidth and ensures the uniformity of the gap field distribution in the output cavity.