The invention discloses an automatic calibrating system for an ultrahigh-frequency partial discharge tester and a method thereof. The system comprises a calibrating signal generator, an UHF partial discharge sensor and a calibrated ultrahigh-frequency partial discharge tester. The ultrahigh-frequency partial discharge tester is connected with the UHF partial discharge sensor. The high-frequency signal output end of the calibrating signal generator is connected with a signal input coaxial joint of a GTEM small chamber. The UHF partial sensor is placed at the central part of the GTEM small chamber. The middle part of the GTEM small chamber is provided with a field strength probe. The field strength probe is connected with an oscilloscope which is arranged outside the GTEM small chamber. Theoutput signal of the oscilloscope is connected with a controller. The controller is connected with the calibrating signal generator. The automatic calibrating system performs real-time hardware part controlling, data acquisition and displaying processing on the ultrahigh-frequency partial discharge tester. The system has advantages of high reliability, real-time measurement and effective calibrating efficiency improvement. A feasible method is supplied for automatic calibration of the ultrahigh-frequency partial discharge tester. The automatic calibrating system further has advantages of highcalibration precision and simple control method.