As　an　excellent　photoelectric　material,　metal　halide　perovskites　have　been　rapidly　developed　in　the　photovoltaic　field.　The　power　conversion　efficiency　of　solar　cells　based　on　perovskite　materials　now　exceeds　24%,　which　is　close　to　the　conversion　efficiency　of　silicon-based　solar　cells.　However,　organic-inorganic　hybrid　perovskite　materials　are　sensitive　to　light,　oxygen,　and　moisture,　particularly　when　combined　in　the　ambient　environment,　limiting　their　commercial　application　in　perovskite　devices　due　to　their　poor　environmental　stability.　Therefore,　a　comprehensive　understanding　of　the　degradation　mechanism　is　the　key　for　development　of　an　effective　method　to　inhibit　the　degradation　of　perovskite　materials.　Herein,　the　photo-induced　degradation　process　of　CH3NH3PbI3　films　in　air　was　studied　by　conventional　optical　and　structural　characterization　methods,　including　ultraviolet-visible　(UV-Vis)　absorption　spectroscopy,　X-ray　diffraction　(XRD)　and　advanced　transmission　electron　microscopy　(TEM)　equipped　with　a　probe　spherical　aberration　corrector.　The　CH3NH3PbI3　films　were　first　decomposed　into　hexagonal　PbI2　and　amorphous　phase,　and　subsequently　oxidized　to　the　amorphous　phase　under　the　combined　effects　of　light　and　oxygen.　The　molecular　formula　of　the　amorphous　phase　was　further　confirmed　as　PbI2-2xOx(0.4　＜　x　＜　0.6)　via　X-ray　energy　dispersive　spectroscopy　(EDS)　and　electron　energy　loss　spectroscopy　(EELS).　Further　analysis　showed　that　the　film　degradation　is　mainly　related　to　superoxide　(O-2(center　dot-))　formed　by　combination　of　oxygen　molecules　and　photoelectrons　in　the　perovskite　film.　The　organic　part　of　the　CH3NH3PbI3　is　oxidized　by　O-2(center　dot-)　and　CH3NH3PbI3　is　decomposed　to　form　volatile　products,　such　as　CH3NH2　and　I-2,　then　degraded　into　PbI2,　and　oxidized　to　form　the　amorphous　PbI2-2xOx.　Therefore,　during　the　initial　degradation　of　film　under　light　soaking　in　air,　the　degradation　sites　are　mainly　located　at　the　interface　between　CH3NH3PbI3　and　air.　Many　pores　were　observed　on　the　film　surface　due　to　the　large　loss　of　volatile　decomposition　products　during　the　initial　degradation.　The　films　then　converted　to　a　honeycomb　hollow　morphology　due　to　the　continuous　consumption　of　material　under　light　soaking,　reducing　the　mass　of　the　film　as　well.　Finally,　the　entire　film　was　oxidized　to　form　an　amorphous　structure.　Herein,　for　the　first　time,　we　report　that　the　formation　of　amorphous　oxides　is　accompanied　by　the　degradation　of　perovskite　film.　This　study　presents　a　new　understanding　of　the　photo-induced　degradation　mechanism　of　perovskite　films　in　air　and　provides　novel　theoretical　guidance　to　promote　the　long-term　stability　of　perovskite　solar　cells.