To enhance the application of ultra-low frequency dielectric loss testing technology in evaluating cable insulation conditions and to improve the testing efficiency of ultra-low frequency dielectric spectroscopy, the dielectric properties of cross-linked polyethylene (XLPE) cable insulation before and after thermal aging are investigated using a improved frequency domain spectroscopy (iFDS). A novel method for characterizing the thermal aging state of cable insulation is proposed. Firstly, XLPE cable insulation samples with varying aging levels are prepared through thermal aging experiments. The dielectric properties of these aged XLPE samples are measured in the ultra-low frequency range from 10⁻³ Hz to 10³ Hz. The results demonstrate that the ultra-low frequency dielectric spectrum is highly sensitive to changes in the XLPE state. As the test frequency decreases, the dielectric loss gradually increases, with more severely aged samples showing a more pronounced rise in dielectric loss. This indicates that ultra-low frequency dielectric loss exhibits strong anti-interference characteristics and can effectively detect XLPE aging conditions. Based on dielectric physics, the sharp increase in dielectric loss in the ultra-low frequency region is primarily attributed to the combined effects of DC conductance and interface polarization. Finally, a correlation between the ultra-low frequency dielectric spectrum and volt-ampere characteristics is established. A volt-ampere hysteresis loop, incorporating amplitude and phase information, is proposed to evaluate cable insulation aging. Relevant characteristic parameters, such as the rate of deflection angle change (RDA) and the rate of curve deformation (RCD), are extracted to characterize the aging status of cable insulation.