Carbon dioxide (CO2) is the raw material of plant photosynthesis and greenhouse gas. Its excessive emission will affect the ecological environment of animals and plants. Under the background of carbon peaking and carbon neutrality, it is of great significance to develop high-sensitivity CO2 detection devices. In order to monitor the change of CO2 concentration in the atmospheric environment, a long optical path and resonant CO2 gas photoacoustic sensor was designed, and a photoacoustic detection setup was built. Adistributed feedback laser (DFB) with a central wavelength of 2 004 nm was used as the excitation light source. The laser entered into a spherical absorption cell made of diffuse reflective material, and multiple reflections occurred in the cell to increase the absorption path of the gas. To reduce the thermal noise generated by the absorption of light energy by the absorption cell, the outside of the cell was wrapped by two aluminum hemispheres with high thermal conductivity. An acoustic tube was coupled with the absorption cell. When the tube worked in the first-order longitudinal resonance mode, the photoacoustic signal was amplified and reached the maximum at the end of the tube. The CO2 relaxation rate was greatly accelerated, and the thermal, acoustic conversion efficiency was improved by saturated humidifying the sample, which further amplified the photoacoustic signal. The photoacoustic signal produced by the humidified sample was about 2.1 times that of the dry sample. The photoacoustic detection setup was calibrated with a series of wet CO2 samples, and the results showed a good linear relationship between photoacoustic signals and concentrations. On this basis, the accuracy and stability of the setup were verified through the detection experiment of standard gas. Allan variance was used to evaluate the detection sensitivity of the setup under long-time operation. When the average time was 865 s, the detection sensitivity was ~0.35×10-6. Compared with the traditional T-type photoacoustic cell, the optical path was increased by~20 times, and the photoacoustic signal was amplified by ~6 times. The setup was used to detect CO2 in the outdoor environment for 10 hours, and the average concentration of outdoor CO2 was ~381×10-6. In conclusion, due to the combination of a long optical path, acoustic resonance and humidified samples, the photoacoustic signal of CO2 was effectively increased, which provided a relevant reference for the design of gas photoacoustic sensor and detection setup.