The greenhouse effect exacerbated by the increase of Carbon-containing gases is the more important causes of the climate change, It is very meaningful to the large-scale flux of carbon dioxide detection for the estimate the contributions of the main greenhouse gases in the atmosphere of various errestrial eco-systems. Tunable diode laser absorption spectroscopy (TDLAS) is a highly sensitive, highly selective and fast time response trace gas detection technique. In the present paper, the authors used a DFB laser was used as the light source, and by employing wavelength modulation method, and measuring the second harmonic signal of one absorption line near 1.573 μm of carbon dioxide molecule, the authors built a system 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 within one day calculated by the formula is within -1.5～2.5, breakking through the phenomenon of only providing the flux of trace gases near the ground at present, makking the measurement of trace gas fluxes within a large area possible.