To address the frequent failure issues caused by defects in the buffer layer of high-voltage cross-linked polyethylene (XLPE) cables, research on buffer layer has become a hot topic at present. This paper first introduces the input impedance model of the 66 kV cable buffer layer and the corrugated aluminum sheath. The water content can change its resistance and capacitance, thereby affecting the resistivity and dielectric constant. Secondly, the paper introduces the intrinsic principles of frequency domain dielectric spectroscopy technology, where changes in water content alter the relaxation time of the Debye model, thereby changing the dielectric loss factor. Then, by conducting resistance and capacitance measurement experiments on a 6 cm long cable buffer layer, the paper derives the resistivity and dielectric constant variation curves. Through dielectric loss factor measurement experiments, it derives the dielectric loss factor curve, demonstrating the correlation between water content and the dielectric loss factor. The paper concludes that the dielectric loss factor curve is more sensitive to insulation conditions at low frequencies.