Abstract: To investigate concentration variation and diffusion characteristics of cross-linking by-products in cross-linked polyethylene (XLPE) insulated cables, a mathematical model based on Fick’s second law was established. A 135 h degassing experiment was conducted on a 110 kV XLPE insulated cable at 65 ℃. Furthermore, to verify relationship between degassing efficiency and dielectric properties of XLPE insulation, simulated degassing tests and dielectric performance tests were carried out on cable samples. Results showed that by-products concentration was closely related to degassing time, degassing temperature, diffusion coefficient, and radial distance within cable. After 135 h degassing experiment at 65 ℃, diffusion coefficients of polar organic by-products and methane were determined to be 4.21×10⁻¹¹ m²·s⁻¹ and 4.95×10⁻¹⁰ m²·s⁻¹, respectively. When simulated degassing test reached 96 h, dielectric constant and dielectric loss tangent of XLPE insulation met the standard requirements, and mass fraction of cross-linking by-products reached its minimum. Total mass fraction of polar organic by-products was 0.216‰, approximately 34% of its pre-degassing value. Based on by-products mass fractions and diffusion coefficients obtained from simulated degassing test, required degassing time for a 110 kV 800 mm² XLPE insulated cable was estimated to be no less than 144 h.