Size　and　crystallinity　of　building　units　in　the　perovskite　layer　are　of　great　significance　to　photovoltaic　performance.　Thus,　to　fabricate　large-grain-size　perovskite　materials　with　the　advantage　of　good　crystallinity　is　quite　necessary.　The　oriented　attachment　strategy　has　been　proofed　as　an　efficient　method　to　control　crystal　growth.　Herein,　we　reported　on　oriented　attachment　of　alpha-CsPbI3　quantum　dots　(QDs)　into　a　large-grain-size　nanocrystal　under　moderate　ultraviolet　(UV)　light.　By　virtue　of　atomic-resolution　TEM　and　X-ray　absorption　fine　structure　(XAFS)　spectroscopy,　we　observed　the　UV-directed　structure-evolution　and　growth　process.　This　is　trigged　by　UV-light　illumination　(7　W,　365　nm),　which　drives　the　oriented　assembly　of　QDs　into　a　large　nanoparticle　along　{110}　facets.　Moreover,　we　also　visualized　a　damage　process　of　the　alpha-CsPbI3　QDs　to　photoinactive-delta-phase　ones　and　finally　into　PIA,　under　high-power　UV-light　(100　W,　365　nm)　exposure.　The　findings　provide　a　prototype　for　fabricating　large-size　perovskite　nanostructures　with　promising　properties.