Herein, we design a carbazole-based COF nanosheet connected by sp 2 carbons, termed JUC-557nanosheet, which is exfoliated from its bulk COF (JUC-557, JUC = Jilin University China). Due to the complementary characteristic of phenyl ethylene (PE) as an aggregation-induced emission (AIE) group and carbazole as an aggregation-caused quenching (ACQ) chromophore in the same framework, JUC-557-nanosheet was proven to have high absolute quantum yields of ~ 23.0% in both solid-state and solution. Therefore, JUC-557-nanosheet as a sensitive fluorescence probe exhibits an excellent molecular recognition toward specific cations and anions as well as iodine (I 2 ) and toxic nitro-compounds, bringing forth I 2 with an association constant (K a ) of 2.10  10 5 M -1 and limit of detection (LOD) of 302 ppb, 2,4,6-trinitrotoluene (TNT) with K a of 4.38  10 5 M -1 and LOD of 129 ppb, and especially nitrobenzene (NB) with K a of 6.18  10 6 M -1 and LOD of 5 ppb, which is much better than those optical detection materials reported until now, including fluorescence porous materials, small molecule probes and inorganics. Furthermore, a combined theoretical and experimental study, including time-resolved photoluminescence (TRPL) measurements, UV-vis absorption spectroscopy, and density functional theory (DFT) theoretical calculations, explains the fluorescence quenching mechanism of JUC-557nanosheet. Due to its high chemical stability, JUC-557-nanosheet also shows excellent luminescence and sensitive molecule recognition even under extreme conditions (e.g., in HCl with pH = 1 or NaOH with pH = 14), and allows optical trace detection of various analytes through the visual detection equipment.