Abstract: At present,oxygen content is generally used as the reference quantity for combustion efficiency monitoring in thermal power units. However,oxygen content can′t accurately reflect the mixing situation of wind coal inthe combustion process,and the oxygen measurement in flue gas is greatly affected by the oxygen in air. Therefore,on the basis of oxygen measurement,and combined with the CO concentration measurement of incomplete combustion,the reliability of combustion efficiency monitoring can be improved.A comparative study of two kinds of CO measuring instruments was carried out,which were realized based on NDIR technology with sampling mode and TDLAS technology with in-suit mode,respectively. The sampling probe rod of NDIR-based with sampling mode instrument was installed in the flue behind the economizer,and the sample gas entered the analyzer for NDIR measurement after the pretreatments which included multistage filtration and condensation and water removal. The measurement chamber of TDLAS-based with in-situ mode instrument was installed in the flue after the economizer,and the flue gas entered the chamber to realize TDLAS measurement after passing through the filter which was installed on the chamber front end. The measurement results show that CO concentration in the flue gas of thermal power units has the characteristics of large variation range and fast change speed. The NDIR-based with sampling mode instrument has a long-time delay due to the sampling path,and it has the shortcoming of large maintenance because of the complex pretreatment such as multi-stage filtration,condensation and water removal,at the same time,the measurement results are easy to be untrue due to the adsorption or reaction characteristics of the gas during the sampling process. The TDLAS-based with in-suit mode instrument is able to respond to the instantaneous change of CO concentration in time due to the short sampling path and fast response speed when the measurement chamber is installed in the flue,and the instrument adopts the integrated structure design,which has high system stability and small maintenance,at the same time,the instrument can realize the high fidelity measurement because the measurement chamber is installed in the flue which can ensure that the measurement environment is consistent with the flue environment.