Polarity is one of the important factors in cell microenvironment. Polarity, to a large extent, is involved in various physiological activities and has an important influence on many life activities of organisms. Studies show that many metabolic diseases in the body can cause polarity changes, such as diabetes, fatty liver, atherosclerosis, and so on. Therefore, it is important to detect polarity changes in living organisms. In this paper, a novel polar fluorescent probe COM-PO with large Stokes-shift was designed and synthesized by altering the push and pull electron groups on coumarin to alter the intramolecular charge transfer (ICT). The fluorescence intensity and wavelength of the probe changed with the polarity of the test system. In polar solvents, the excited state energy of COM-PO dissipates in the solvent through dipole-dipole interaction, and its fluorescence emission intensity decreases. On the contrary, the fluorescence emission intensity of the probe increases significantly in low polar solvents. Since the probe has a large Stokes shift, it can avoid the relatively high signal-to-noise ratio and the serious fluorescence self-extinguishing in the imaging process. Therefore, we applied the probe to cell diagnosis and in vivo disease diagnosis. It is well known that cancer cells are less polar than normal cells, and when the probe enters cancer cells, their fluorescence intensity increases by two to three times compared to normal cells. The fluorescence intensity of the probe COM-PO was also twice as high as that of the normal mice. The COM-PO probe was tested by solution spectroscopy, cell imaging, tissue imaging and mouse in vivo imaging. The experimental results show that the probe COM-PO has the advantages of high polarity sensitivity, high selectivity, strong pH adaptability and good biocompatibility, and can detect the polarity changes of abnormal states of cells, tissues and even livers, which is expected to realize the early diagnosis of polarity related diseases.