Abstract: Combined with in-situ sampling, the WM-DAS method was used to realize fast on-line measurement of CO volume fraction in flue gas of coal-fired units aiming at its strong fluctuation characteristics. Firstly, the WM-DAS method was theoretically deduced and verified. Then, the corresponding device was installed on the horizontal flue duct at the exit of the economizer in a coal-fired power plant, while the measuring chamber was installed in the flue duct. Under the suction effect of Rafael nozzle, the flue gas entered the measuring chamber after being filtered by the filter in the front of the measuring chamber to realize the measurement. Results show that the on-line CO measurement device based on in-situ sampling can achieve fast and high-precision on-line measurement of CO volume fraction. The CO volume fraction is highly correlated with that of the O₂.