The utility model relates to a voltage transformer local discharge ultrahigh frequency (UHF) online monitoring device. The device comprises an ultrahigh frequency sensor, an ultrahigh frequency coaxial feeder, a signal hardware regulating unit, a high frequency coaxial cable, an A/D collecting card and an industrial control computer. The ultrahigh frequency sensor and the end cap of the flange of a manhole (hand hole) are integrally arranged on the manhole (hand hole) of the wall of a voltage transformer oil tank and are positioned in the voltage transformer. The utility model overcomes the disadvantage that when the sensor is arranged through the accident fuel outlet valve of the voltage transformer or the leak valve at the top of the oil tank, because the arranging position and the dimension are limited, and the effective detecting range is small, the sensor capability is sacrificed. The signal is regulated by the signal hardware regulating unit, which can more effectively abstract a discharging signal in the entire detecting band and enhance the detecting sensibility.