ln this paper.the Finite Diffenence Time-Domam method (FDTD method) is employed for the first tlme to andyse the septum polarizer of antenna system m airborne nacelle jammer deslgned by ourselves in detail. The performance parameters and distinct physical images of this new type of polarizer are yielded. And a new kind of annota tlOn of polarizer IS gl ven ,w hich is easy to understand.