An air-silica microstructure optical fiber based on the anti-resonant reflecting optical waveguide (ARROW) principle was used to develop a spectral absorption gas sensor. The ARROW fiber has an air core and an air cladding layer. An ARROW fiber with a length of 725 mm was used to construct a sensing system to detect acetylene gas. The gas was injected into the fiber from one end of the fiber. The transmission spectra were collected using an optical spectrum analyzer. The results indicate that the system can detect the gas of different concentrations and has the good system linearity. The response time of the system is about 200 s.