Abstract: To study the concentration variation and diffusion characteristics of crosslinking byproducts in XLPE insulated cables, a mathematical model for byproduct concentration based on Ficks Second Law was established. Real degassing experiments and simulated degassing tests were conducted on 110 kV XLPE insulated cables at 65 °C. By measuring the byproduct concentration variation and dielectric properties of the cable samples, the influence of degassing time on the relative dielectric constant and loss tangent of the cables under power frequency conditions was thoroughly investigated. The results show that the byproduct concentration is closely related to degassing time, degassing temperature, diffusion coefficient, and the radial distance along the cable. Under the conditions of 135 hours degassing time at 65 °C, the diffusion coefficient of polar byproducts in 110 kV 800 mm² insulated cables was 1.67×10⁻⁹ m²/s, while that of methane gas was 3.70×10⁻⁸ m²/s. The simulated degassing experiments indicate that when the degassing time reaches 84 hours, the dielectric constant and loss tangent of the cables meet the national standard requirements. For 110 kV 800 mm² XLPE insulated cables, it is recommended that the degassing time should not be less than 142 hours.