Abstract: Simulation and experiments of the output system for an L-band 10 MW multi-beam klystron are carried out. Parameters of the two-port six-beam coaxial output cavity, such as frequency, quality factor and character impedance, are calculated and analyzed. A output window with a nonstandard flat waveguide is designed, in which two metallic poles are adopted to match the impedance. The VSWR of the window is lower than 1.01 at 1.3 GHz, and the bandwidth is greater than 100 MHz with the (Voltage Standing Wave Ratio) VSWR lower than 1.2. A testing model is manufactured and measured. The experimental results agree well with the design. The beam-wave interaction characteristics of the output cavity are simulated by a 3-D PIC software, and the output radio frequency power is greater than 10 MW with efficiency of more than 65%.