The invention discloses a high-temperature insulating material for a heater. The high-temperature insulating material is prepared from the following ingredients in parts by weight: 75-90 parts of silicon rubber, 2-5 parts of calcium hydroxide, 5-10 parts of magnesia, 4-6 parts of calcium carbonate, 0.4-1.1 parts of polyethylene wax, 7-11 parts of carbon black and 1-3 parts of chlorinated paraffin. According to a preparation method of the high-temperature insulating material, silicon rubber is adopted as a main material, so that the high-temperature insulating material has a good high temperature resisting effect while excellent insulating performance is guaranteed, and the service life of a product is prolonged; and due to the addition of high-activity magnesia, the improvement on the activity of the material can be facilitated, and the use effect is enhanced. The high-temperature insulating material for the heater, prepared by the preparation method, has very good high temperature resistance, and the use temperature can reach 150-180 DEG C. By adopting the formula of the high-temperature insulating material, production resources of enterprises can be saved greatly, and the production cost for the enterprises is reduced.