The invention discloses a calibration method for an ultrahigh-frequency partial discharge online monitoring device in a field environment. Firstly, connection between a monitored sensor and a measurement host is kept, ultrahigh-frequency electromagnetic wave signals are injected to the monitored sensor through a signal generator, and the measurement host acquires measured response signals of the monitored online monitoring device; then, the injected ultrahigh-frequency electromagnetic wave signals are kept unchanged, electrical connection between the monitored sensor and the measurement host is cut off, a radio frequency cable is adopted to be connected with a measurement module with standard measurement, and standard response signals of the monitored sensor are acquired; and finally, through the standard response signals and the measured response signals, a calibration result is generated. According to the calibration method provided by the invention, the calibration efficiency is improved, the human errors are reduced, the confidence of calibration data is improved, and influences on the test by factors such as distribution parameters introduced by frequent wiring, the connection reliability degree and external interference can be reduced.