Abstract: Epoxy resin and alumina/epoxy resin composite materials are widely used in electrical equipment insulation. However, due to the existence of hydrophilic groups such as hydroxyl groups, epoxy resin is easy to absorb moisture and cause insulation deterioration. In this paper, considering the influence of temperature and voltage on the frequency domain spectrum (FDS) curve is taken into consideration, and the FDS characteristics of epoxy resin under different moisture content are studied. The results show that the shift degree of the FDS curve is larger in the high frequency band with the increase of temperature while smaller in the low frequency band. Therefore, a piecewise fitting method is proposed to reduce the evaluation error caused by different high and low frequency activation energy. The voltage basically does not affect the high frequency range of the FDS curve. The influence of voltage on the low frequency band has three areas (stable area, restricted area, saturation area), and the dielectric loss value in the restricted area gradually decreases with the increase of voltage. When the moisture decreases, the low-frequency amplitude of the FDS curve of pure epoxy resin decreases significantly, while the amplitude of the alumina/epoxy resin curve decreases in the entire frequency band. The influence of moisture on the tanδ of the two types of insulation is the most remarkable at 0.01 Hz, thus this frequency can be used as the characteristic frequency for diagnosing the damp defects.