Aiming at the problem that radar is easy to be damaged when being attacked by HPM weapons， this paper theoretically analyzes the relationship between the farthest protection boundary to antenna gain and receiving front-end limiter’s capability， and compares the capability of reflector radar and active phased array radar against front door coupling attack from the perspective of spatial filtering. The analysis shows that only when the HPM weapon is near the main lobe range of the reflector radar antenna， its front door coupling attack effect is stronger than that of the active phased array radar； on the contrary， it is weaker than active phased array radar. Because the main beam of the reflector antenna radar is very narrow， the attack time window is very short， so the reflector antenna radar has stronger protection ability when facing HPM weapons.