We investigate the effect of pressure on the electronic spin state of ferric iron on Al-bearing MgSiO3-perovskite using first-principle computations. Ferric iron (6.25 mol%) and Al (6.25 mol%) substitute for Mg and Si respectively. Five substitution models on different atomic position pairs are examined. Our results show that spin state transition from high spin (HS) to low spin (LS) occurs on the Fe3+ ions at high pressure, while there is no stability field for the intermediate spin state. Fe3+ alone can be responsible for the spin state transition. The five models witness a transition pressure ranging from 97--126 GPa. Differential stress can change the pressure for the spin collapse. The lowest pressure spin state transition occurs where Al3+ and Fe3+ are in adjacent sites. These results are one explanation to the reported experimental observations that the spin transition occurs over a wide pressure range. This finding may have important implications for the dynamics and seismic signature of the lower mantle.