Cross-linked polyethylene (XLPE) cables generate cross-linking by-products such as acetophenone, cumyl alcohol, and α-methylstyrene during the manufacturing process. Under the influence of an electric field, these by-products dissociate, forming space charges that lead to the initiation and growth of electrical treeing, thereby reducing the insulation performance of the cables. Degassing treatment is considered one of the effective methods for reducing cross-linking by-products. However, the current selection of degassing time mainly relies on experience and lacks a unified standard. The effect of degassing time on the initiation and growth characteristics of electrical treeing in XLPE cables is experimentally investigated. Gas chromatography-mass spectrometry (GC-MS) is used to quantitatively analyze the changes in the volume fraction of cross-linking by-products. In conjunction with electrical treeing initiation and growth experiments, the mechanism by which cross-linking by-products influence the growth of DC electrical treeing is explored. The experimental results show that degassing treatment has a significant effect on reducing the volume fraction of all three cross-linking by-products, with α-methylstyrene exhibiting the most pronounced removal. The rate of decline in cross-linking by-product volume fraction varies at different stages of degassing. The reduction in cross-linking by-products makes the initiation and growth of electrical treeing more difficult. In practical industrial production, degassing should ensure that the volume fraction of cross-linking by-products is reduced to less than 20% of the initial level. A theoretical basis is provided for optimizing the degassing process of XLPE cables, and a reasonable degassing time is recommended for use in actual production to minimize cross-linking by-products, thereby enhancing the insulation performance and service life of the cables.