Abstract: Three kinds of domestic LDPE before and after improvement for high voltage cable insulation are tested by means of GPC and NMR. The structural parameters such as molecular weight and distribution, double bond content and long-chain branching degree are compared and analyzed. The DC electrical properties of LDPE and cross-linked polyethylene are tested. The results show that the improved LDPE has a larger molecular weight, a narrower molecular distribution, a certain increase in double bond content and long-chain branching degree. There are significant improvements in electrical properties such as volume resistivity, temperature coefficient of resistivity, and DC breakdown strength. The overall volume resistivity of the XLPE prepared by the improved LDPE is one order of magnitude higher, and the volume resistivity at 90 ℃ is two orders of magnitude higher. The temperature coefficient of resistivity is greatly reduced, and the DC breakdown strength is increased by about 30%.