Photonic crystal was first proposed as a new kind of material in1980s as it can control the movement of the photon which moves in it. In this article, we proposed and demonstrated the theory and method that photonic crystal waveguide (PCW) producing slow light, and slow light enhancing the light–matter interaction and gas molar absorption coefficient. On the basis of this, we proposed and designed a new type of optical fiber gas sensor by utilizing PCW and slow light produced by the PCW. Finally, through theory analysis, we conclude that the sensitivity of this new fiber optic sensor increases 12.56 times and the volume of the system unit is also significantly reduced. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1796–1800, 2013