Abstract: The phenomenon of waste of energy caused by heat leak exists in the industry such as heat-engine plant, metallurgy and petrochemical industry and so on. On the basis of commenting on the defects of the traditional method of measuring heat loss, for example, heat current test, conducted heat transfer test, convection heat transfer test and enthalpy drop test, a kind of method using infrared thermo-graphic diagnostic technique to measure heat loss was proposed, which could scientifically evaluate heat insulation effect of thermal power equipment. By actual measurement and calculating for cylindrical thermal reactor of petrochemical works of Jilin, the direct economic loss caused by heat leak that exceeded the stipulated standards was obtained. The results showed that the method had a wide range of application.