Capturing　the　photolysis　process　of　archetypal　perovskite　materials　(such　as　methylammonium　lead　triiodide,　MAPbI(3))　into　metallic　Pb-0　is　fundamentally　important　to　understand　the　photodegradation　pathway　of　organic-inorganic　hybrid　perovskites　(OIHPs).　In　this　study,　the　photodecomposition　pathway　of　a　MAPbI(3)　film　in　an　atmospheric　argon　(Ar)　gas　environment　was　quasi-in　situ　studied　comprehensively.　Under　light　illumination,　the　decomposition　of　the　perovskite　film　is　first　triggered　at　the　grain　boundaries,　along　with　an　obvious　generation　of　metallic　lead　(Pb-0)　nanoparticles.　Then,　the　degradation　sites　extend　into　the　inner　MAPbI(3)　grains.　By　calculating　the　growth　kinetics　of　the　Pb-0　enriched　nanoparticles,　we　can　find　that　it　strictly　obeys　an　interfacial　diffusion-controlled　growth　model.　Atomic　resolution　high　angle　annular　dark-field　(HAADF)　characterization　confirms　that　the　Pb-0　enriched　nanoparticles　are　with　Pb@PbI2-x　(x　approximate　to　0.4)　core-shell　structures,　while　the　interface　between　crystalline　MAPbI(3)　and　amorphous　PbI2-x　controls　the　photodegradation　pathway　of　perovskite　films.　To　the　best　of　our　knowledge,　this　is　the　first　time　to　directly　observe　the　photolysis　process　of　perovskite　films　in　a　practical　environment　(Ar　gas　and　light　illumination),　particularly　with　nano　or　even　atomic-scale　resolution.　Furthermore,　the　discovery　of　the　interfacial-controlled　photodegradation　pathway　of　MAPbI(3)　into　amorphous　PbI2-x　and　then　into　Pb@PbI2-x　aggregates　has　never　been　elucidated　before.