The invention discloses a radiation-resistant halogen-free low smoke flame retardant insulating material for cables of nuclear power plants and a preparation method. The radiation-resistant halogen-free low smoke flame retardant insulating material is prepared from the following raw materials: ethylene propylene diene rubber, phenyl siloxane rubber, magnesium hydrate, aluminium hydroxide, zinc borate, white carbon black, cerium dioxide, zinc oxide, Bis(tert-butyldioxyisopropyl)benzene, tri allyl cyanurate, 2-mercaptobenzimidazole, 2,2,4-trimethyl-1,2-dihydroquinoline, Aflux A-25, stearic acid, vinyl tris(beta-methoxyethoxy) silane, and paraffin oil by heating and mixing. The formulation and the mixture ratio are scientific and reasonable, and the process is advanced. The prepared insulating material has good radiation resistance, excellent mechanical property, good electrical property and long service life, and is a good radiation-resistant halogen-free low smoke flame retardant insulating material for manufacturing the cables of the nuclear power plants.