The applications of atmospheric carbon isotope are becoming more and more important, which are used widely in the transfer tracer of environmental pollution and the geochemical development. So it is significative to develop novel technologies for carbon isotope detection. Currently, the laser absorption spectroscopy compared with other techniques, which has great advantages of small volume, and high sensitivity which realises real-time online testing,. A large number of researchers have paid close attention to gas isotope probing with laser absorption spectroscopy. In this paper, the tunable semiconductor laser DFB of 2.7 μm wavelength is studied as to the performance. At the same time, the absorption lines of 12CO2 and 13CO2 isotopic molecules are chosen according to selection principles of the characteristics and the isotope molecules absorption line. On the basis, the appropriate wavelength from the laser output is determined. Afterwards, the δ13C isotopic abundances of the CO2 molecule are probed in the atmosphere by the laser absorption spectroscopy under the help of the new multi-pass cell with the 690.3 m optical path length.