Abstract: Based on the demand for a 3GW direct current offshore wind power project, in order to meet the market demands at home and abroad, the design and test verification of high operating temperature ±550kV cross-linked polyethylene insulated direct current submarine cables have been carried out. Characterization of fundamental dielectric properties—including space charge behavior, high-field DC conductivity, and DC breakdown strength—was performed on the 90°C DC cable insulation material to verify its reliability. This investigation established the functional relationship between conductivity and (electric field, temperature), along with electric field—temperature/thickness characteristic curves, thereby providing a theoretical foundation for DC cable structural design. Based on the test results of the dielectric properties of insulating materials, complete the design of the cable structure, conduct research on degassing processes, and study the factory joint processes. At an operating temperature of 90℃, calculations of transmission capacity, electric field analysis were carried out. In accordance with the requirements of GB/T 31489.1—2015 and CIGRE TB852:2021 standards, full-performance type test verification was conducted at 90℃.