The structure, electronic and optical properties of inorganic perovskite materials CsBI₃ (B= Pb, Sn, Ge) are analyzed based on the first principle by Siesta software. First, a stable material structure is obtained using the GGA-PBE and GGA-PBEsol methods. Then, the bandgap of the material is analyzed based on two density functional methods GGA-PBE and GGA-BLYP. This work simulates the material strain by changing the lattice constant of the material, showing that the bandgap increases with an increase in the lattice constant. Moreover, doping with Ge can decrease the bandgap of supercell CsPbI_3. When doped with low-concentration Ge, the band gap of the material was found to reduce by 0.7% to 3.8%. Finally, from the absorption spectrum, it can be seen that the absorption coefficients of CsPbI₃ and CsGeI₃ are close to 6×10⁵ cm^-1, and the absorption peak of CsPbI₃ is approximately 350 nm and that of CsGeI₃ is approximately 410 nm. However, the absorption coefficient of CsSnI₃ is close to 4.75×10⁵ cm^-1, and the absorption peak is approximately 350 nm.