Combustion of power fuel in a large thermal power plant produces a great deal of harmful flue gas, which endangers personal safety of residents around it. In this paper, the injury mechanism of flue gas ejected by the thermal power plant is analyzed. The injury indexes of flue gas ejected by the thermal power plant are put forward. In order to improve the safety status of flue gas ejected by the thermal power plant, we developed a special instrument, which can detect on-line the injury indexes of flue gas ingredients, particle species, ejection speed and ejection flux in the ejected flue gas. The detection data can be used to assess the safety statue of the ejected flue gas, to predict the injury degree of the ejected flue gas to people and to provide basic data for establishing the valid emergency schemes.