Abstract: By use of such theories as lossy transmission line, microwave equivalent circuits, network matrix of electromagnetism and so on, this paper analyses the process of energy storage and energy dumping of the cavity of the excessive modes cylindrical cavity pulse compress system, and gains the theoretic calculating formula of power gain, which is consistent with experiment result. Consequently the formula is proved to be reliable and feasible perfect direct the effect on the design, manufacture, and debugging of the compress system. Moreover, it is beneficial to develop deeper research.