The flue gas emitted from stationary pollution sources has greatly increased pollution to the environment , and it is very important to scientifically monitor the flue gas. In the paper, the authors introduce the status of the on-line flue gas monitoring systems, including automatic air quality monitoring system (AQMS), continuous emission monitoring system (CEMS) based on differential optical absorption spectroscopy(DOAS), on-line wireless sensor network monitoring system under the internet of thing .The structure of every on-line flue gas monitoring system, technical specification, practical applications and development direction have been described. And the relationship between the on-line gas monitoring systems and meteorological systems is discussed in this paper.