CO_2 concentration has ranked the third of all the trace gas in the earth's atmosphere and exerts a tremendous influence on greenhouse effect. It mainly comes from human's daily activities. Detecting atmospheric CO_2 concentration has important significance in understanding the. periodic changes of CO_2 and the inherent law of climate change. Based on tunable diode laser absorption spectroscopy (TDLAS) technology, the absorption line of CO_2 near 2004 nm was selected, the open-path atmospheric CO_2 was continuously measured by direct absorption processing method. Compared with a hand-held CO_2 measurement instrument, their correlation coefficient is 0.8371, and the standard deviation is 7.204 ppm. The results show good consistency between the TDLAS and hand-held CO_2 measurement instrument, and the feasibility of experiment system is proved. It provides an important way and a method for the domestic remote sensing of CO_2 with laser.