Methylamine　(CH3NH2,　MA)　gas-induced　fabrication　of　organometal　CH3NH3PbI3　based　perovskite　thin　films　are　promising　photovoltaic　materials　that　transform　the　energy　from　absorbed　sunlight　into　electrical　power.　Unfortunately,　the　low　stability　of　the　perovskites　poses　a　serious　hindrance　for　further　development,　compared　to　conventional　inorganic　materials.　The　solid-state　perovskites　are　liquefied　and　recrystallized　from　CH3NH2.　However,　the　mechanism　of　this　phase　transformation　is　far　from　clear.　Employing　first　principles　calculations　and　ab　initio　molecular　dynamics　simulations,　we　investigated　the　formation　energy　of　primary　defects　in　perovskites　and　the　liquefaction　process　in　CH3NH2　vapor.　The　results　indicated　that　defect-assisted　surface　dissolution　leads　to　the　liquefaction　of　perovskite　thin　films　in　CH3NH2　vapor.　Two　primary　defects　were　studied:　one　is　the　Frenkel　pair　defect　(including　both　negatively　charged　interstitial　iodide　ion　(I-i(-))　and　iodide　vacancy　(V-I(+))　at　the　PbI2-termination　surface,　and　the　other　is　the　Schottky　defects　(methylammonium　vacancy,　V-MA)　at　the　MAI-termination　surface.　Moreover,　the　defect-induced　disorder　in　the　microstructure　reduces　the　degeneration　of　energy　levels,　which　leads　to　a　blue　shift　and　broader　absorption　band　gap,　as　compared　to　the　clean　perovskite　surface.　The　mechanism　of　how　defects　impact　the　surface　dissolution　could　be　applied　for　the　further　design　of　high-stability　perovskite　solar　cells.