Abstract: Multipath interference, caused by sea water reflection, will lead to elevation lobing of the radar, which antenna has widebeam, and observation blind space and tracking target losing. This paper focuses on antenna and electromagnetic wave propagation, analyzing the 3D pattern of the interrogation antenna of the monopulse secondary surveillance radar over sea water, and introducing the principle of the antenna beam interference. This paper also analyzes the interferential azimuth and elevation patterns ofthe slantwise sea-base linear phased array antenna, discusses the angle measurement errors caused by pattern deformation, andconclude some countermeasures to reduce the affection.