Photoacoustic spectroscopy technology is a new type of spectral analys is detection technology based on the photoacoustic effect,which has the advanta ges of fast response,short measurement time,and high detection sensitivity.An d it has become a fast,safe and reliable method for trace gas detection.In thi s paper,a distributed feedback tunable diode laser with a center wavelength of 2.0μm was used as the light source and a photoacoustic spectrometric system fo r measuring carbon dioxide (CO₂) had been developed.In the experiment,the sampl e gas was humidified.And the relaxation time of CO_2molecules was reduced by i ntroducing H₂O molecules with fast relaxation rate,which can accelerate the re la xation rate of CO_2molecules and then improve the photoacoustic signal of CO 2g as and the detection sensitivity of the system.The CO_2absorption spectrum lin e at 4998.35cm-1was selected as the research object in the measurement,combine d with wavelength modulation technology,the CO_2gas was detected and studied.Fi rstly,the optimal modulation frequency and amplitude of the system were determi ned to be 785Hz and 7.586cm－1 by evaluating the system.Then,it was foun d tha t there was a good linear relationship between the measured photoacoustic signal and the concentration of CO_2gas by measuring a series of CO₂-N_2mixed s ample g ases under the optimal experimental conditions.And then the concentration of CO _2in the real atmosphere was measured by this system.And the content of CO_2i n the outdoor atmosphere was about 3.82×10－4.Finally,this system was als o used to continuously measure the CO_2content in the outdoor atmosphere for 24conse cutive hours,and obtained the concentration change of CO_2gas in a day.And the long-term stability of the system was analyzed by using Allan Variance.The dete c tion sensitivity was about 1×10－7 when the average time of the system wa s 1000s,which proved that the system had good stability and detection sensitivity.