Abstract: Based on the problems of benzene tank leakage in a chemical factory, by the important analysis methods and the proliferated law of benzene, four zones were divided according to the lethal low concentration, harmful concentration, concentration of severe poisoning and concentration of minor poisoning. The chain of equipment failure of the main reasons for system to tank leakage was established, and four forewarning grades were divided. By GIS, the forewarning system of benzene storage tank leakage was developed. The feasibility of forewarning technique was verified in Henan Shenma Nylon Chemical Co., Ltd.