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The outstanding specifications of klystron are high output power, high gain, high reliability, but its transient band width (BW) is narrow, so its applications are limited. To broad the BW of klystron, double gap cavity, several gap cavity, traveling wave klystron and multi-beam klystron (MBK) have been invented. In them the MBK has low working voltage, broad transient BW, so it gets more and more attention. According to microwave theory and electron-magnet filed theory, the final BW of klystron is limited in the BW of group collection segment and BW of power output segment. Usually the BW of group collection segment is broader than the BW of power output segment, the MBK's group collection segment has more broad BW. So the key problem of broad klystron's BW is how to broad the BW of power output system. So people invented double cavity, several cavity distribution cavity structure, overlap model output, output cavity add filter and so on. The structures of output cavity add filter is simple and easy to be made. Many MBK use this structure. In past when people design this structure, first split this problem into two parts, one is output cavity design, the other is filter design; use many approximate conditions, use microwave circuit method to design it, the result can't be accurate then to make cold test model, many times change, many times test, after many periods the results can be use on the practice klystron. The working cycle is long, the efficient is low. At recent times, many microwave compute software appears and get more and more mature, such as MAFIA, HFSS program. Use these software people can simulate the electro-magnet filed in microwave tubes, also can compute the interaction and energy interchange of cell and field. In practice people use these software can quickly get the better design ,greatly improve the design level, short the design time. In this text ,use MAFIA HFSS compute the single cavity, double gap cavity, single cavity add filter structure, double gap cavity add filter (13 ∼ 18GHz), from the compute results, we can see that the double cavity add filter is better than other structure. And analysis all the factors of this structure. Then made a cold-test model, the test results is basically according to the compute - results, It has 2.5% cold-test BW. In practice klystron this structure has 1.6% hot-test BW (Ku band, 20kW). In this text conclusion of CAD design in broad BW is made. Also discuss some skills and problems of using the software.