The effect of pressure (up to 10 GPa) on the electronic and optical properties of bromine-substituted cesium lead iodide (CsPbI3), as one promising inorganic halide perovskite, is investigated using modified Backe-Johnson (mBJ) potential for the first time to our knowledge. The lattice parameters, electronic bandgap, and imaginary and real parts of the dielectric function, along with the optical absorption coefficient, are calculated. Density functional perturbation theory is employed to compute the optical properties in the photon energy range from 0.0 to 30 eV. No structural or phase-type transformation is noticed under the applied pressure, which resulted in a uniform contraction of the unit cell. Bandgap variation is seen in all the structures, with the maximum (1.65 eV) and minimum (1.46 eV) decrease found for doped and undoped CsPbBrI2, respectively. The present work provides useful information about the performance of CsPbI3-xBrx compounds under high pressure that can be utilized in designing solar cells and optoelectronics.