Abstract: In order to study the effect of different degassing time on the content of cross-linking by-products, the insulation properties and aggregate structure of 500 kV high voltage direct current XLPE, 500 kV cross-linked polyethylene (XLPE) cable material was used to prepare samples by flat-plate hot pressing in this paper. Four samples with different degassing time of 0h, 12h, 36h and 90h respectively were obtained by degassing treatment at 70℃ at normal pressure. Fourier transform infrared (FTIR), weightlessness experiment, thermos gravimetric analysis (TGA), current density, space charge, DC breakdown, X-ray diffraction (XRD) test and scanning electron microscope (SEM) observation were carried out. Thereafter, a crystal growth model of XLPE was constructed, the related mechanism of material insulation characteristics and microstructure changes during degassing treatment was analyzed. The results show that the cross-linking by-products are almost completely volatilized after degassing for 660min. The theoretical calculation of Fick's second law can reflect the change trend of the residual content of cross-linking by-products. The increase of degassing duration is beneficial to increasing the breakdown field strength of the sample and reducing the electrical conductivity current density of the sample. However, there is an infection point at 36h; the degassing treatment helps to improve the space charge accumulation and electric field distortion inside the sample, reduce the dispersion of the sample's DC breakdown and the difference in the internal aggregation structure and increases the thickness of the lamellae. The above results indicate that the degassing treatment is helpful for the volatilization of the crosslinking by-products and reasonable optimization of the degassing time can improve the insulation properties of XLPE, thereby reducing the production and time costs of the enterprise.