The invention provides a nano composite crosslinked polyethylene insulating material with high direct current breakdown field strength and a preparation method of the nano composite crosslinked polyethylene insulating material. The insulating material is prepared from the following components in parts by mass: 100 parts of low density polyethylene, 0.1 to 3 parts of nano boron nitride particles, 1 to 2.5 parts of a crosslinking agent and 0.1 to 0.5 part of an antioxidant. The insulating material prepared based on the preparation method can be used as an insulating material for a direct current cable; compared with crosslinked polyethylene without the nano boron nitride particles, the nano composite crosslinked polyethylene insulating material has the advantage that the direct current breakdown field strength is obviously improved; at 20 DEG C, 70 DEG C and 90 DEG C, the direct current breakdown field strength is improved by 27.5 percent, 14.2 percent and 26.5 percent respectively; the nano composite crosslinked polyethylene insulating material is relatively high in direct current breakdown field strength.