The gas sensing property of high-index-core Bragg fiber is investigated in this paper via a powerful full-vector finite element method. Based on the simulation and analysis, the relationship between the gas sensing property of high-index-core Bragg fiber and the fiber structure parameters, as well as the wavelength, has been gained, which shows the gas sensing property of the fibers increases with the decrease of the fiber's radius and refractive index, but it increases with the increase of the wavelength. These results will be helpful to the design of high performance one-dimensional microstructured gas or liquid sensing fibers.