The　development　of　small-molecule　organic　fluorophores　in　the　second　near-infrared　region　(NIR-II,　1000-1700　nm)　has　recently　received　immense　attention　due　to　deep　tissue　light　penetration　and　reduced　autofluorescence.　Polymethine　cyanine　dyes　as　important　building　blocks　are　frequently　used　to　design　NIR-II　light-activated　fluomphores.　However,　applications　of　these　fluomphores　were　restricted　by　their　poor　photostability.　Herein,　we　synthesized　a　NIR-II　heptamethine　cyanine　dye　4　as　the　model　compound　to　investigate　its　photostability　and　identify　its　photoproducts.　Compound　4　shows　improved　photostability　compared　to　ICG,　and　remains　relatively　stable　even　under　sunlight　irradiation.　The　cyanine　scaffold＇s　good　photostability　promoted　its　stable　photothermal　properties.　The　effective　photothermal　activity　of　cyanine　dye　4　and　the　low　toxicity　of　its　one　major　photoproduct　demonstrated　its　potential　for　further　photo-therapeutic　applications.　This　work　would　pave　the　way　for　the　development　of　NIR-II　cyanines　with　photostability　through　delicate　structure　design.