Abstract: In order to get high sensitivity detection of volume fraction of carbon dioxide gas, a carbon dioxide gas sensor based on the principle of infrared spectrum absorption was designed by using 9m hollow-core photonic crystal fiber as sensing unit. The absorption spectra of carbon dioxide gas with different volume fractions were measured under the same absorption wavelength. The analysis results show that the linear relationship between gas absorption intensity and volume fraction of gas is presented, in accordance with Beer-Lambert law. The detective sensitivity of the system is estimated to be 4.389×10^-5W. It can make the system get higher sensitivity by lengthening the photonic crystal fiber and increasing the effective distance of gas absorption.