Theoretical　studies　have　shown　that　surface　terminations,　such　as　MAI　or　PbI　layers,　greatly　affect　the　environmental　stability　of　organic-inorganic　perovskite.　However,　until　now,　there　has　been　little　effort　to　experimentally　detect　the　existence　of　MAI　or　PbI　terminations　on　MAPbI3　grains,　let　alone　disclose　their　effects　on　the　humidity　degradation　pathway　of　perovskite　solar　cell.　Here,　we　successfully　modified　and　detected　the　surface　terminations　of　MAI　and　PbI　species　on　polycrystalline　MAPbI3　films.　MAI-terminated　perovskite　film　followed　the　moisture　degradation　process　from　MAPbI3　to　hydrate　MAPbI3　⋅H2　O　and　then　into　PbI2　,　with　penetration　of　water　molecules　being　the　main　driving　force　leading　to　the　degradation　of　MAPbI3　layer　by　layer.　In　contrast,　for　the　PbI-terminated　perovskite　film　in　a　humid　atmosphere,　a　deprotonation　degradation　pathway　was　confirmed,　in　which　the　film　preferentially　degraded　directly　from　MAPbI3　into　PbI2　,　here　the　iodine　defects　played　a　key　role　in　promoting　the　dissociation　of　water　molecules　into　OH-　and　further　catalyzing　the　decomposition　of　perovskite.