The key techniques for the high-sensitivity detection of CO₂ in the 2 μm band are studied based on photo-thermal spectroscopy in a hollow-core optical fiber, and a low-sharpness Fabry-Perot interferometer based on anti-resonance hollow-core optical fiber is adopted to demodulate the photo-thermal phase. For the R(18) absorption line of CO₂ at 2004.02 nm, the noise equivalent volume fraction is about 4.7×10 ^-8 with a pump power of 180 mW and integration time of 1 s. To improve the stability of the detection system, we suppress the intra-cavity interference of pump light by applying the anti-reflection coating to the fiber end face and lock the probe wavelength to the quadrature point of the interferometer. Within 1 h, the signal fluctuation corresponding to CO₂ standard gas with volume fraction of 10 ^-5 is about 4.7%. Noise analysis shows that the noise mainly comes from the phase noise and intensity noise of the probe laser in the current detection system.