Long-term operation or external forces may cause damage to the outer sheath of the cable, allowing moisture to enter the cable body and affecting the insulation and dielectric performance of the main XLPE insulation of the cable. For 66 kV cables, there are no clear values provided for the changes in the dielectric loss tangent and capacitance after water ingress into the main insulation. Using a self-built experimental platform, dielectric spectroscopy curves were plotted for the main insulation under 1 kV voltage, showing the variation of the dielectric loss tangent with water content and frequency. The unit capacitance changes of the main insulation due to moisture were also measured. The study indicates that at 0.01 Hz, when the water content in the main insulation increases from 0.86% to 1.29%, the dielectric loss tangent value changes by several times. Additionally, this value is not affected by the location of the moisture—whether it is accumulated, evenly distributed, or symmetrically distributed, the dielectric loss tangent remains unchanged. When the volumetric water content increases by 0.48%, the unit capacitance value increases by 0.8%.