A carbon dioxide (CO2) gas sensor based on a polyhexamethylene biguanide (PHMB)-coated whispering gallery mode (WGM) microbubble resonator is proposed and verified experimentally in this work. Microbubbles were fabricated using two reverse arc discharges focused on microcapillaries. The inner wall of the microbubble was coated with a layer of PHMB using a filling and sintering process. A significant WGM resonance was observed by coupling with a tapered fiber. The experimental results show that as the concentration of carbon dioxide increases, a blue shift appears in the spectrum. Addition, a high sensitivity (0.46 pm /ppm) and a good linear relationship were obtained in the measurement range of 200-700 ppm with a detection limit of 50 ppm. The sensor features include high sensitivity, simple structure, easy manufacture, and low cost.