Tunable diode laser absorption spectroscopy (TDLAS) technique is a new method to detect trace gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectra in the characteristic absorption region. TDLAS is a highly sensitive, highly selective and fast time response trace gas detection technique. In the present paper, a DFB laser at room temperature was used as the light source, wavelength modulation method was employed, and the second harmonic signal of one absorption line near 1.578 μm of carbon dioxide molecule was measured. A system was built for online monitoring of carbon dioxide concentration within the optical path of more than 700 meters at different heights. Combined with Alonzo Mourning-Obukhov length and characteristic velocity detected by large aperture scintillometer, the flux of carbon dioxide gas calculated by the experiential formula is within -60~60 mg·m-2·s-1. The comparison of the datea detected by TDLAS system and the eddy covariance showed that the change of the data detected by TDLAS had a similar trend to that detected by the eddy covariance, and the best results can be produced by this method, breakking through the phenomenon of only providing the flux of trace gases near the ground at present, and making the measurement of trace gas fluxes within a large area possible.