Tunable diode laser absorption spectroscopy(TDLAS) is a new method to detect trace-gas qualitatively or quantitatively based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. With the combination of TDLAS and the open-path multi-pass Herriot cell, using second harmonic detection and the auto-balanced detection with wavelength modulation respectively, the CO concentration of flame of alcohol blowtorch is measured. The result shows that, comparing with second harmonic detection, the new detection method of auto-balanced detection with wavelength modulation not only improves detectability of 16.3 times, but also eliminates impact from light-intensity fluctuation generated by laser and flame effectively, and could be applied for CO concentration monitoring in many fields including combustion control and component measurement of flame.