The invention belongs to the field of insulating layers of transmission equipment and particularly relates to a grafted crosslinked polyethylene insulating layer for a high voltage direct current cable and a preparation method thereof. By taking a thermoplastic polyethylene material as a base body, a polyethylene composition containing graftable polar groups can be prepared by proportioning raw materials provided by the invention. After the composition is melted and extruded in an extruder, the composition is chemically crosslinked through a crosslinking pipeline, and small molecules with polar groups are grafted to the molecular chains thereof while a polyethylene material is crosslinked to obtain the grafted crosslinked polyethylene insulating layer, wherein the spatial charge can be inhibited obviously. The polar group grafted crosslinked polyethylene insulating layer provided by the invention is applied to the high voltage direct current cable, so that the problem of charge accumulation in the transmission material under the action of the direct current electric field can be inhibited. The grafted crosslinked polyethylene insulating layer prepared by the preparation method notonly is excellent in spatial load inhibiting property, but also has relatively high direct current disruptive strength and relatively low conductivity.