Abnormal level of alkaline phosphatase (ALP) activity has been linked to many diseases in human. The development of fluorescent molecular probes that can report the expression and activity of ALP in various biological systems will be extremely valuable. However, the in vivo monitoring for ALP in living cells and more complex biological systems remains a great challenge. The excited-state intramolecular proton transfer (ESIPT) probe with proportional fluorescence has low background noise, while the aggregation induced emission (AIE) probe has the advantages of signal amplification and good light stability. Herein, an "AIE + ESIPT" fluorescent probe 2-(benzo[d]thiazol-2-yl)-4-(1,4,5-triphenyl-1H-imidazole-2-yl)phenyl dihydrogen phosphate (THP) was constructed for the highly selective and sensitive detection of ALP. By introducing a phosphate ester at the hydroxyl position of the solid fluorophore 2-(benzo[d]thiazol-2-yl)-4-(1,4,5-triphenyl-1H-imidazole-2-yl)phenol, ESIPT was hindered and the probe present a faint blue fluorescence in DMSO solution. While ALP was introduced, causing the phosphate in THP hydrolyzed, and the ESIPT process was restored to yield a yellow fluorescence at 550 nm, thereby achieving proportionality detection. THP exhibited high selectivity and sensitively to ALP with low limit of detection (1.21228 U/L), and the reaction completed within 20 min. In addition, with its outstanding advantages of low biological toxicity and enzyme conversion characteristics, THP has been successfully applied to ALP imaging in living cells (Hela cells, A549 cells and Hek293 cells), and can provide in situ information on the reaction site. Therefore, THP has the potential for detecting ALP activity in biomedical application.