A Fabry–Perot interferometer (FPI) for CO<sub>2</sub> gas sensing at atmospheric pressure is proposed and experimentally demonstrated. The gas sensing material is poly(ethyleneimine) (PEI)/poly(vinylalcohol) (PVA) compound, which exhibits reversible refrative index change upon absorption and release of CO<sub>2</sub> gas molecules. The FPI is fabricated by coating a PEI/P VA film with a thickness of 15μm film at the end face of a single-mode fiber (SMF). A well-confined interference spectrum with fringe contrast of 19.5 dB and free spectra range (FSR) of 33.15 nm is obtained. The proposed FPI sensor is sensitive to the CO<sub>2</sub> gas concentration change, and a sensitivity of 0.2833nm/PCT is obtained. The FPI sensor provides a solution in the development of low-cost and compact gas sensors for CO<sub>2</sub> leakage monitoring.