The continuous automatic monitoring system (CEMS) for pollution source flue gas adopts a unique dilution technology, which has significant advantages compared to various direct sampling techniques. The constant control of the dilution ratio of the probe eliminates the need for heating or temperature and pressure compensation. The dilution and control parts are simple and clear, and completely eliminate the effects of system corrosion and blockage. According to the requirements of the 1990 Clean Air Act in the United States, dilution method is the preferred method for online detection of pollution sources. Among the more than 2000 pollution source systems installed in the United States, more than 1800 use dilution method, of which more than 1600 use Thermo Fisher Scientific systems. In China, Thermo Scientific not only provides the first dilution system, but also occupies the majority of the domestic dilution method market.