The invention discloses an insulating material for producing a mineral insulated cable and a preparation method for the insulating material. The insulating material is mainly prepared from the following raw materials: fly ash, potassium tetrafluoroaluminate and magnesia. The preparation method comprises the steps: mixing the fly ash, the potassium tetrafluoroaluminate and the magnesia in a stirrer, and carrying out thorough uniform mixing; spraying an aqueous solution into the powder mixture, and carrying out mixing, so as to enable the powder mixture to have certain humidity and bonding property; pressing the mixed powder material into porcelain knobs of required shapes by a knob presser; and placing the porcelain knobs into a burning furnace, carrying out heated calcination for 2.5 to 3hours at the temperature of 1,180 DEG C to 1,200 DEG C, naturally cooling the temperature of the burning furnace DEG C to 40 DEG C to 50 DEG C, and carrying out furnace discharging, thereby obtainingthe insulating material. The problem that the magnesia absorbs water and is poor in insulation, difficult in shaping and poor in uniformity is solved. The prepared insulating material is good in uniformity and high in insulation resistance.