CO as the incomplete combustion product in flue gas of coalfired unit is an important basis for combustion efficiency monitoring, which has the characteristics of high concentration, strong fluctuation and uneven distribution. In this paper, the mixed sampling online measurement of CO concentration in flue gas is carried out using wavelength modulation spectroscopy in TDLAS technology combined with dilution method. Firstly, based on the harmonic theory, the measurement formulas of the fourth harmonic and first harmonic calibrationfree method were deduced. Secondly, the phase difference between frequency modulation and light intensity modulation, the current tuning coefficient of the CO laser were calibrated, and the optical path of Herriott cell was measured through standard gas combined with direct absorption method. On this basis, the curve of concentration versus harmonic ratio was numerically calculated and verified in experiment. Finally, the monitoring device was applied in a coalfired power plant. During installation, the outlet flues of the economizer were divided into six areas, and a dilution sampling probe was installed in each area. During the dilution sampling process, because the dew point of flue gas was reduced, no dewatering was needed. The diluted flue gases in the six channels were mixed and then entered into the Herriott cell to realize the calibrationfree and highprecision measurement. The monitoring device realized continuous and stable operation, and in the operation Testo350 portable analyzer was used to make measurement and the measurement results were compared with the measurement data obtained from the field measurement device. The comparison results show that the two measurement results are consistent and the error between them is less than 1×10-6.