Abstract: Blast furnace is a high temperature reactor which includes complex physical chemistry reaction and transportation process. The model of partical reaction, section balance of heat and materials can not fit in quick changes of condition of BF. The theorem on fuzzy mathematics is applied to deal with uncentain knowledge. From this, this knowledge base and reasoning mechanism are formed. Furtherly, the expert system based on AI is established for judging the state of BF. Expert system can refrain from the negligence of tiny changes of information by even excellent operator.