In order to prolong the working time of calcined flint clay–bauxite castables during construction at high temperature, boric acid was added into the castables. The effect of boric acid on working time and curing cold crushing strength of the castables at 25 ℃ and 35 ℃ were investigated. After 24 h curing in mould and another 24 h curing at 110 ℃ after demoulding, the specimens were heat treated at 1 000 ℃, 1 300 ℃, and 1 500 ℃ for 3 h, respectively. The permanent linear change, bulk density, modulus of rupture, and cold crushing strength were determined. The result shows that there is no need to add boric acid when calcined flint clay-bauxite castables works at 25 ℃; when calcined flint clay-bauxite castables works at 35 ℃, boric acid can increase the working time of the castables, but decrease the curing cold crushing strength a little. Adding boric acid into calcined flint clay–bauxite castables doesn’t worsen performance of the castables.