The higher viscosity and lower pH in lysosomes of cancer cells highlight their potential as biomarkers for cancer. Therefore, the development of acid-activated viscosity fluorescent probes is significant for the early diagnosis and treatment of cancer. Based on this, we have designed and synthesized a near-infrared fluorescent probe based on the 2-(2-hydroxyphenyl)benzothiazole (HBT) group, namely HBTH, to monitor the viscosity changes within lysosomes. It has been demonstrated that HBTH was extremely sensitive to viscosity, with a strong linear relationship between fluorescence intensity and log(viscosity) within the range of (logη) = 0-3.06 (a correlation coefficient of 0.98), proving its capability for quantitative viscosity measurement. In particular, the most obvious fluorescence enhancement of HBTH was only efficiently triggered by the combined effect of low pH and high viscosity. Furthermore, HBTH can rapidly localize to lysosomes by wash-free procedure at a low concentration (100 nM) and achieve high-fidelity imaging within 20 s. It can also monitor the dynamic processes of lysosomes in cells, viscosity changes under drug stimuli, and lysosomal behavior during mitophagy. Importantly, HBTH is capable of identifying tumors in tumor-bearing nude mice through in vivo imaging. These features make HBTH a powerful tool for the early diagnosis and treatment of cancer.