Optical waveguide has a promising application as absorption cell in concentration measurement for gases and liquids. Hollow fiber has many advantages when it is used as the absorption cell for spectroscopic gas sensing. The characteristics of the sensing system are directly related to various parameters of the hollow fiber. A mathematical model is proposed considering both the transmission loss of the hollow fiber and the signal-to-noise ratio of the system. Theoretical analysis is made for the dependence of the output intensity and sensitivity on the fiber parameters such as length and inner-diameter, the concentration of target gas, as well as the intensity distribution of light source. Several important results are obtained from the simulation, which provide optimizing methods for the sensing system and algorithms for error compensation.