The invention relates to an energy-saving slag thermal insulator, and belongs to the technical field of energy conservation, emission reduction and comprehensive utilization in the metallurgy industry. The thermal insulator comprises the following main chemical components in percentage by weight: 25 to 70 percent of SiO2, 1 to 25 percent of Al2O3, 1 to 40 percent of CaO, 0 to 20 percent of MgO, 0 to 10 percent of F<->, 0 to 10 percent of R2O (R is a monovalent metal ion), 0 to 10 percent of Fe2O3, 5 to 60 percent of fixed carbon with bulk density of 0.25 to 0.75g/cm<3>. The thermal insulator has high thermal insulation performance and can reduce the thermal loss of high-temperature slag; the thermal insulator contains a certain amount of fixed carbon, so heat generated in the chemical reaction of the fixed carbon can be used for further making up the thermal loss of the slag; meanwhile, the thermal insulator belongs to a silicon-aluminum material, and part or all of the thermal insulator can be dissolved in the slag finally so as to improve the chemical composition of the slag and reduce the melting point of the slag. The thermal insulator has the advantages of simple production process, low cost, and suitability for various processes for utilizing the sensible heat of the slag, increasing the fluidity of the slag or improving the performance of the slag.