The invention provides a high voltage direct current cable insulating material and a preparation method thereof, wherein the high voltage direct current cable insulating material comprises a main material and an auxiliary material, the main material is a low density polyethylene resin, and the auxiliary material comprises a polystyrene resin, a cross-linking agent and an antioxidant. Compared withthe common cross-linked polyethylene, the high voltage direct current cable insulating material of the invention has the following characteristics that the direct current breakdown field strength isincreased within the temperature range of 30-90 DEG C, the direct current breakdown field strength is improved by 72.9% at the temperature of 70 DEG C, and the direct current breakdown field strengthis improved by 43.7% at the temperature of 90 DEG C.