The invention provides an insulating material for a flame-retardant cable and a preparation method of the insulating material. The preparation method includes of firstly, mixing and stirring tetrafluoroethylene-hexafluoropropylene copolymer, butyl rubber, ethylene propylene diene monomer, zinc N-ethyl-N-phenyldithiocarbamate, plasticizer, organic solvent and graphite to prepare a mixture; secondly, conducting extruding, forming and cooling on the mixture to prepare particles, and preparing the insulating material. The insulating cable prepared through the method has excellent insulating resistance, the preparation method is simple, and raw materials are easy to obtain.