The crystal structures of BiOI under different pressures are predicted, inwhich the phase transformation from tetragonal BiOI into orthorhombic BiOIis observed at 100 GPa due to the appearance of Bi–I bonds (Bi-6s2lone pairand I-pzobital) between interlayer Bi and I atoms. Most interestingly, fororthorhombic BiOI, an isostructural electronic phase separation is observedat 160 GPa. The electronic structure of BiOI separates into semiconductorstate (contributed by intralayer O–Bi–O) and metal state, which originatefrom the interlayer I–I bonds. More specifically, conversion from interlayerBi–I interaction to interlayer I–I interaction under higher pressure, results inthe continuous decline of the I-dx2–y2energy band, which then crosses theFermi-level. In comparison, the O–Bi–O layer shows almost no change,owing to its strong covalent bonds. These results demonstrate that thedramatic variation of the interaction between the van der Waals layers canlead to the metallic iodine character in BiOI.