Absorption spectroscopy based on mid-infrared light sources is one of the effective methods for CO2 gas detection and analysis, and it has significant potential applications in environmental science and medical fields. However, most of the light sources used for the detection of CO2 in the 2.8 μm band are commercial diode lasers or semiconductor lasers, which have a single output wavelength and poor beam quality, and the laser output power is small, which is susceptible to environmental fluctuations during the gas testing process. In this paper, a CO2 gas detection system based on a mid-infrared fiber laser light source is constructed, which uses a 980 nm semiconductor laser as the pump light source and 1.9 m Er3+∶ZBLAN fiber as the gain medium to realize a high-quality mid-infrared laser output at 2.8 μm. It analyzes the output characteristics of the light source under this structure. When the maximum pump power is 6 W, the center wavelength of the laser output spectrum is 2 783.09 nm, the spectral width is 5.69 nm, the average output power is 1.21 W, and the conversion efficiency is 20.3%. The output power fluctuation of the laser is less than 4.5% within 30 min, and the output energy is relatively concentrated. The fitted beam waist radii Dx and Dy are 0.035 mm and 0.038 mm, and the beam quality factors M2x and M2y are 1.096 and 1.224, respectively. The better quality and stability of the lasers output beam dramatically reduces the effects of environmental fluctuations during gas detection. On this basis, the signal spectra before and after CO2 fluxing were obtained using direct absorption spectroscopy. The center wavelengths of the absorption peaks of CO2 in the 2 778.5~2 784 nm band were measured to be 2 779.36, 2 780.70, 2 782.13, 2 783.55 nm, and the difference in the center wavelengths of the absorption peaks with the HITRAN standard library was less than 0.05 nm. The peak and half-height width of the CO2 absorption spectrum at the center wavelength of 2 780.7 nm was 5.33 cm·mol-1 and 0.18 nm, obtained by inversion of the Lambert-Beer law. The corresponding peak absorption spectra and half-height width values of the HITRAN standard library under the same conditions were 5.38 cm·mol-1 and 0.12 nm, respectively, with a difference of only 0.05 cm·mol-1 and 0.06 nm. The high accuracy of the experimental test results proves that the detection system can be used in CO2 gas detection.