Abstract: This paper presents a filter protecting the radio astronomicalfrequency band,which is a novel coaxial band-rejection filter in structure.The filter uses λ/4 coaxial cavities as resonater and round wire in squareouter conducter as transmission line.In designing of the filter by makinggood use of design technology of coaxial filters,the measured performanceof high attenuation in stopband and low insertion loss in passband issatisfactorily obtained.