Organic-inorganic　perovskite　single　crystals　are　promising　in　the　field　of　optoelectronics　due　to　their　excellent　optoelectronic　properties.　However,　the　ion　transport　of　perovskite　precursor　is　poor　in　confined　spaces,　which　results　in　difficulty　in　the　preparation　of　perovskite　single-crystal　films.　Herein,　MAPbBr3　films　consisting　of　square　grains　were　fabricated　by　the　surface-confined　process　using　the　organic　molecule　PEAI　(phenethylammonium　iodide).　Under　the　effect　of　oversaturation　gradient,　PEA+　is　combined　with　the　surface　of　perovskite　grain　from　top　to　side,　which　constrains　the　lateral　growth　of　grains　and　induces　a　downward　growth　of　perovskite,　leading　to　the　formation　of　square　grains.　With　the　improvement　of　concentration　PEAI,　the　perovskite　film　exhibits　a　decreased　side　length　of　grains　(from　0.98　to　12.96　μm)　and　increased　grain　number　and　coverage,　as　well　as　crystallinity.　The　perovskite　single　crystalline　grain　films　with　PEAI　showed　double　photoluminescence　(PL)　emission　peaks　due　to　the　existence　of　iodine-rich　perovskite.　This　work　may　provide　a　practical　way　to　fabricate　high-quality　perovskite　films　for　perovskite　photoelectronic　devices.