Viscosity is an important part of the cell microenvironment. Abnormal viscosity in the microenvironment is related to many diseases such as tumors, fatty liver and Alzheimer's disease. Accumulated β-amyloid (Aβ) fibrils and plaques are key pathological features of Alzheimer's disease, which can lead to severe neurotoxicity and cognitive and behavioral disorders. In this work, a near-infrared fluorescent probe (G) based on anthraquinone-quinolinium dye was constructed to detect viscosity. When the viscosity of the medium is high, the free rotation of the single bond is suppressed and the fluorescence intensity is enhanced. The probe shows high selectivity for viscosity. It is worth noting that probe G has a long excitation wavelength (640 nm) and emission wavelength (825 nm) with a large Stocks shift of 185 nm. At the same time, probe G is a positively charged molecule that can target mitochondria. In addition, probe G can be not only used to detect the viscosity of Aβ protein in vitro, but also image the abnormal viscosity of Aβ protein in the brain of Alzheimer's disease mice.