The high-pressure behaviors of crystalline (Ba0.5Sr0.5)TiO3 (BST) are investigated, using the first-principles calculations based on the density functional theory. The results show that as pressure increases, the band gap of BST first increases and peaks at around 55 GPa, and then gradually decreases. The analysis of density of states shows that in the low-pressure region (0P55 GPa), the increase in band gap is due to the formation of anti-bonding states and bonding states in the conduction band and valence band, respectively. In the high-pressure region (P55 GPa), the delocalization phenomenon in dominant due to the fact that the delocaligation action exceeds the force of bonding state and anti-bonding state, which results in the decrease of the band gap.