Ultra high frequency (UHF) partial discharge detection technology has been widely used in on-line monitoring of electrical equipment. However, the calibration of UHF method is still not realized until now, because the influence factors of UHF signal on transfer function are complicated. In order to study the calibration influence factors of UHF partial discharge (PD) detector, the discharge mechanism of typical PD defects is analyzed, and the relative experimental research in propagation characteristics and sensor characteristics of UHF signals are performed. It is concluded that significant difference is found between UHF signal energy and discharge capacity of different discharge source for calibration reliability. The calibration curve of corona discharge and suspended discharge represents the severity of equipment insulation defect more accurately, while the calibration curve of internal air gap discharge and dielectric surface discharge does ineffectively. The distance of UHF signal energy decays to stable period decreases with the increase of frequency, and the decay of UHF signal energy is irrelevant to its frequencies by measuring in different directions. Therefore, in order to calibration the UHF signal, it is necessary to consider the specific discharge type and measurement condition comprehensively. The results provide the favorable reference for further studies to build calibration system of UHF measurement, and promote effective application of UHF method in sensor characteristic fault diagnosis and insulation evaluation of electrical equipment.