Based on the density functional theory, we investigate the effects of stacking patterns and interlayer coupling on the stability, electronic structure and optical properties of bilayer (BL) BiI3 by utilizing first-principles methods. The stability and electronic structure are significantly influenced by the stacking patterns and interlayer distances. Moreover, the results show that the most stable pattern has the smallest equilibrium interlayer distance among all eight considered stacking cases. Although the nature of band gap of each stacking pattern is indirect band gap, interestingly, for seven considered patterns, the transition from indirect to direct occurs when the interlayer distance is below the equilibrium distance. The optical properties of BL BiI3 are sensitive to interlayer distances.