Absorption gratings are the key devices in grating-based X-ray phase-contrast imaging (XPCI). The low cost and fitness for the fabrication in general laboratories make bismuth absorption grating favored. A calculating method for fringe visibility in bismuth grating-based XPCI is proposed, and the moire fringe visibilities of bismuth absorption gratings with different thicknesses are calculated through modeling. Results show that fringe visibility increases with the increasing thickness of bismuth structure. The fringe visibility for π phase grating can reach 48% under the 40 kV tube voltage, but only 22% under 60 kV, when the thicknesses of bismuth structure in source gratings and analyzer gratings are 150 μm and 110 μm, respectively. Furthermore, when the bismuth structure thicknesses of the two absorption gratings are equivalent, fringe visibilities are obtained by use of the π phase and π/2 phase gratings, respectively. Their quantitative comparison shows that the result of employing π/2 phase grating is slightly better than that of π phase grating. The quantitative calculation of fringe visibility will be beneficial to the design of grating-based XPCI system, which may promote the practicality of this technology.