In summary, we have successfully obtained a carbazole-based sp 2 carbon COF nanosheet, JUC-557nanosheet, with a thickness about 2.5 nm by exfoliating the corresponding pristine bulk powder. Owing to the synergistic effect of the AIE-molecular rotors (PE) and ACQ-based chromophore units (carbazole), JUC-557-nanosheet demonstrates high absolute quantum yields (up to 23.0%) in both solid-state and solution. Also, electron-rich carbazole and electron-deficient cyan are uniformly distributed and exposed on the channel wall, rendering efficient combining with the analytes for detection. Therefore, JUC-557nanosheet as a sensitive fluorescence sensor exhibits an excellent molecular recognition toward the essential element in the human body (Fe 3+ with K a of 1.98  10 4 M -1 and LOD of 706 ppb), radioactive contaminant (I 2 with K a of 2.10  10 5 M -1 and LOD of 302 ppb), explosive (TNT with K a of 4.38  10 5 M -1 and LOD of 129 ppb), and especially toxic nitro-compounds (such as NB with K a of 6.18  10 6 M -1 and LOD of 5 ppb), which is far superior to previous fluorescence detection materials, including COFs, MOFs, POPs, small molecule probes, and inorganics. Due to its high chemical stability, JUC-557nanosheet exhibits a good luminescence and sensitive molecule recognition even under extreme conditions, such as in strong acid with pH = 1 or strong base with pH = 14. Moreover, JUC-557-nanosheet can be assembled into visual detection equipment, allowing easy optical trace detection of various analytes. Finally, the molecular recognition mechanism was investigated by a combined theoretical and experimental studies, including TRPL measurements, UV-vis absorption spectroscopy, and DFT calculations. Our study thus promotes the development of stable fluorescence COF nanosheets as novel versatile sensing platforms that could be integrated with electronic devices for environment monitoring.