Cross-linked polyethylene power cables are widely used in high-voltage DC transmission lines, owing to their good dielectric and physical-chemical properties. However, the production process of XLPE involves cross-linking and degassing, in which the cross-linking process produces a variety of cross-linking by-products, and the changes in the properties of the cable insulation caused by the degassing process are not well understood. XLPE samples were degassed at 90 °C for 7 and 14 days in this paper, and the main by-products were found to be α-methylstyrene, acetophenone, and cumyl alcohol, the contents of which all declined after the degassing treatment. The results show that the space charge density, the leakage current under a high electric field at different temperatures, and the breakdown strength of the XLPE samples all decreased after the degassing treatment. On the other hand, the XLPE sample after 7 days' degassing had the lowest conductivity and the highest conductance activation, and the space charge density and the charge decay rate as well as the breakdown strength after 7 days' degassing differed little from the 14-day treated sample, demonstrating that the 7-day degassing treatment at 90 °C would be enough to achieve superior performance.