Grain　boundaries　(GBs)　have　a　significant　role　in　polycrystalline　perovskite　solar　cells　(PSCs).　However,　there　is　ongoing　debate　regarding　the　impact　of　GBs　on　the　performance　and　long-term　stability　of　PSCs.　Employing　the　first-principles　molecular　dynamics　for　perovskites,　the　iodine　vacancy　defect　migrations　both　in　bulk　and　at　GBs　are　investigated.　i)　The　positive　iodine　vacancy　(VI+)　is　found　that　have　both　lower　formation　energy　(1.4　eV)　and　activation　energy　(0.18　eV)　than　those　of　neutral　iodine　vacancy　(VI),　statistically.　It　indicated　the　VI+　acts　as　the　dominant　migrated　iodine　vacancy　rather　than　VI;　ii)　the　iodine　vacancy　at　GBs　has　approximate　to　0.48　eV　higher　activation　energy　than　those　in　bulk,　which　leads　to　the　accumulation　of　iodine　vacancy　at　GBs;　iii)　the　presence　of　VI+　result　in　a　3-fold　increase　in　charge　recombination　ratio　at　GBs,　compared　to　pristine　PSCs.　Based　on　quantum　molecular　dynamics　statistical　results,　which　are　consistent　with　experimental　measurements,　insights　into　iodine　vacancy　migration　both　at　GBs　and　in　the　bulk　are　gained.　This　understanding　can　be　valuable　for　defects　engineering　related　to　ion　migration,　in　order　to　improve　the　long-term　stability　and　promote　the　performance　of　PSCs.　Understanding　defects　engineering　related　to　ion　migration　is　crucial　for　enhancing　the　long-term　stability　and　performance　of　hybrid　perovskite　solar　cells.　Iodine　vacancies　accumulate　at　grain　boundaries　due　to　lower　formation　energy　and　higher　migration　potential　barrier　compared　to　those　in　the　bulk,　which　further　increase　the　nonradiative　recombination.　image