Fluorescence　(FL)　imaging　in　the　second　nearinfrared　window　(NIR-II,　1000-1700　nm)　has　emerged　as　a　promising　bioimaging　modality　that　enables　noninvasive　visual-　ization　of　deep　tissue　with　an　unprecedented　resolution.　However,　there　is　a　paucity　of　studies　on　high-quality　responsive　NIR-II　FL　molecules.　Herein　we　report　a　novel　activated　NIR-II　FL　molecule,　4,7-bis(5-(4-(diphenylamine)phenyI)-2-thiophene)　[1,2,5]selenadiazolo[3,4-f]benzo[c][1,2,5]-thiadiazole　(SeTT),　which　exhibits　fast　and　specific　responsive　capability　to　hypochlorous　acid　(HCIO).　To　obtain　the　NIR-II　ratiometric　nanoprobe,　SeTT　was　encapsulated　on　the　surface　of　Er3+-doped　down-conversion　nanoparticles　(DCNP),　achieving　the　DCNP@SeTT　nanoprobe.　With　a　single　980　nm　laser　excitation,　the　ratiometric　FL　signal　of　SeTT　at　1150　nm　and　DCNP　at　1550　nm　(I-1150　(nm)/I-1550　(nm))　was　linearly　correlated　with　the　concentration　of　HCIO　with　a　detection　limit　of　0.4　mu　M.　The　ratiometric　nanoprobe　was　successfully　investigated　for　variations　in　HClO　concentration　in　the　tumor　progression,　visualization　of　anatomical　structures　of　the　peritoneal　cavity　in　the　mice　model　with　inflammation,　and　quantitative　detection　of　the　HClO　concentration　in　a　rabbit　model　of　osteoarthritis,　achieving　a　fast　response　and　high　selectivity　for　the　detection　of　HClO.　The　NIR-II-responsive　nanoprobe　can　serve　as　a　promising　and　effective　tool　for　highly　sensitive　monitoring　and　imaging　of　HClO　in　living　systems.