With the investment of power electronic devices in the power grid and the increasing use of nonlinear loads, the level of high-order harmonics in the system is constantly improving. High-order harmonics not only have a negative impact on the behavior of equipment and materials but also affect the online testing results of insulation parameters. Therefore, the dielectric loss characteristics (tan δ) of cable insulation under cyclic aging and online monitoring correction methods are studied through theoretical analysis and experimental testing. The dielectric loss factor of cross-linked polyethylene cables after aging in the frequency range of 1?8 times the power frequency under different voltages is tested. The research results indicate that as the degree of aging increases, the tan δ value increases, with the largest increase observed in the early stage of 20 aging treatments. For the 50 Hz condition, the tan δ value increases from 0.009?4 to 0.021?9. The degree of increase in tan δ is influenced by harmonic frequency and harmonic voltage, and the influence of harmonic frequency cannot be ignored. Based on the experimental results, a method for correcting the dielectric loss factor of cable insulation is proposed, providing correction factors at high frequencies of 50?400 Hz. The research results can be used for correcting online dielectric loss monitoring under power frequency conditions, thereby improving the reliability of online evaluation of cable aging.