It is an ultra-resistant casting material specially used in tertiary air duct elbow which is a mixture of the following materials in the specialized mass fraction parts. 25-35 parts of alumina particles in 10-3mm; 15-25 parts of the alumina particles in 3-1mm; 23-36 parts of the mixture of the carbon silicon particles; 10-15 parts of mullite powder in 320 mesh; 4-6 parts of silica fume in 0.8 micron and 5-8 parts of cement, add the right amount of concrete water reducer, anti-knock additive and water; then proportionally mixing the above ingredients and adding to the brickwork cavity and vibrating dense, finally the duct castable can be produced after baking. The duct castable significantly shorten the baking time, prevent baking damage and solve the problem of high maintenance which caused by the short service life of the tertiary air duct or castable refractory.