Abstract: Monopulse antenna is one of the most exact electronic directional technologies now, and it is extensively used in microwave and millimeter wave tracking, surveillance, communication, measurement and astronomical system. However, in order to protect the radar antenna from environmental influence, lots of monopulse antennas all use radome, which seriously affects on antenna polarization characteristic. In addition, intentional electronic deception jamming such as cross polarization jamming has great impact on the directional performance of monopulse radar. Taking the commonly used amplitude compari-son monopulse antenna of X band radar seeker as the object, the mechanism of structure, characteristics, primary source of paraboloid radome which induces cross polarization is studied. Calculation model with typical physical parameters is established. The cross polarization characteristics of monopulse antenna with radome under Ludwig Ⅲ definition is simulated. Some key factors such as antenna geometry structure, offset angle, antenna scanning is considered. Simulation results show that, many factors may cause the significant depolarization effect, while the complex polarization pattern structure make orientation accuracy of radar seek sensitive to the polarization of waves. This study provides an important theoretical basis for ECM and ECCM technology studies in monopulse tracking area.