The inherent spin-orbit coupling (SOC) effect in non-centrosymmetric crystal structure has laid the foundation of Rashba splitting phenomena. This Rashba splitting directly governs the charge carrier recombination, which eventually controls the carrier lifetime and diffusion length and therefore the solar cell efficiency for such hybrid perovskite materials. In this work, we have performed a rigorous structural search prediction of the mixed cation-mixed halide hybrid perovskites FA0.83MA0.17Pb(I0.83Br0.17)3 and FA0.875MA0.125Pb(I0.875Br0.125)3, which are the two nearest neighbor structures of record efficiency (22.1%) holder FA0.85MA0.15Pb(I0.85Br0.15)3 in the structural composition phase space. We have found the prediction routes for a structural search such as the mixed perovskite structure govern the Rashba splitting energy value, depending on whether it has been predicted from FPI (FAPbI3) or MPB (MAPbBr3) as parent structure, which are leading to the mixed phase FA0.83MA0.17Pb(I0.83Br0.17)3 and FA0.875MA0.125Pb(I0.875Br,0.125)3 respectively. The strong dependency of the splitting energy on the structural phase evolution along with the stoichiometry and space group is also observed, where in the mixed phase, 0.045 difference in concentration could lead to a remarkable difference in the splitting energy, which is more pronounced in the valence band as compared to the conduction band. We have also determined the Goldschmidt tolerance factor to envisage structural stability of the newly predicted crystal structures based on the corresponding chemical route in the composition phase space. Copyright © 2019 American Chemical Society.