Abstract: A distributed frequency domain dielectric spectrum measurement method based on Havriliak-Negami model is studied to evaluate the wet state of casing. Based on the distributed frequency domain dielectric spectrum measurement method, the variation of dielectric loss of casing model with different water content under 0.1 Hz and 1 Hz frequency and the variation of dielectric loss tangent value of inner and outer layer wetted casing are analyzed. The frequency domain dielectric spectrum parameters of oil immersed casing with different water content are fitted by Havriliak-Negami model. The results show that with the increase of voltage, the higher the water content, the greater the linear increase range of low frequency dielectric loss of casing; the relaxation time constant decreases with the increase of water content, and the moisture content meets the logarithmic linear relationship; the capacitance between the end shield joint and the secondary end shield of the casing is not always sensitive to the moisture in the outer layer of the casing; the capacitance between the guide rod and the secondary end shield is very sensitive to the moisture in the inner layer of the casing, determining the damp location is of great significance to the diagnosis of casing damp.