Belonging to the technical field of cable terminal discharge monitoring, the utility model relates to an ultrahigh frequency sensor for on-line monitoring of cable partial discharge so as to solve the problems that existing ultra wideband frequency-independent antenna does not have small transmission loss and cannot ensure characteristics of a low frequency end. The ultrahigh frequency sensor includes an antenna substrate, equiangular spiral antenna segments, Archimedes antenna segments, a microstrip line substrate, a central conduction band microstrip line and an earth plate microstrip line. The equiangular spiral antenna segments and the Archimedes antenna segments are spirally arranged on the antenna substrate from the center to the outer ring direction in order, and two tail ends of the equiangular spiral antenna segments are in one-to-one corresponding connection with two head ends of the Archimedes antenna segments. The center of the head end side of the microstrip line substrate is equipped with a protruding rectangular connecting column. The central conduction band microstrip line covers the front center of the microstrip line substrate from the head end to the tail end, and the earth plate microstrip line covers the reverse center of the microstrip line substrate from the head end to the tail end. The utility model is used for on-line monitoring of ultra high frequency signals of cable partial discharge.