It has been reported that frequency domain spectroscopy (FDS) is sensitive to moisture content for oil-paper insulation. In order to reflect the characteristics of frequency domain dielectric spectrum about damp-affected defect in transformer oil paper insulation bushings, comparative study for the influence of voltage value and frequency value on dielectric loss from 0.001 Hz to 1 kHz is carried out. The results show that the value of tan δ increases slightly with time at the same value of test voltage in the first stage of the process of rising damp. At the same time, an increase of the value of tan δ is seen with the increasing test voltage. In the second stage of the process of rising damp, the value of tan δ increases remarkably with time, and reaches its peak at the time point of 120 h. At the third stage of the process of rising damp, the value of tan δ decreases with time. The value of tan δ in the whole process does not exceed the values of standard, and the value of standard is suggested to be reduced. In the low frequency band from 0.001 to 0.01 Hz, it is easy to distinguish the damp-affected bushings from the non-damp-affected bushings by frequency domain dielectric spectrum, especially at the frequency of 1 mHz. Therefore, it is recommended that FDS low-frequency test is used for insulation diagnosis.