The invention provides insulating rubber for an ultrahigh-voltage direct-current cable accessory and a preparation method thereof. The insulating rubber is prepared from the following raw materials: ethylene propylene diene monomer, zinc oxide, stearic acid, an anti-aging agent, a plasticizer, a reinforcing agent, a nano filler modifier, a silane coupling agent, a vulcanizing agent and a vulcanization accelerator. By adjusting the dosage of the nano-filler modifier, changing the trap depth in the insulating material, adsorbing free charges in the material, reducing the number of the free charges and reducing charge accumulation, electric field distortion is reduced, electric field distribution is more uniform, effective control over volume resistivity is achieved, the electrical resistivity of the insulating material in a stress cone and the electrical resistivity of a cable insulating material reach the optimal matching state under different environment use temperature conditions. Meanwhile, electrical, mechanical, aging and forming properties and the like meet the technical index requirements, and the material can be applied to the field of ultrahigh-voltage direct-current cableinsulating materials.