Compared with the traditional approaches, UV-Vis spectroscopy is gaining growing acceptance in the field of online water-quality monitoring as an effective method for water quality analysis. UV-Vis spectroscopy has the advantages of fast response, low maintenance costs, and no secondary pollution. In the present paper, the principle of water quality analysis by UV-Vis spectroscopy is introduced. Existing researches on the online analysis of water quality using UV-Vis spectrum and data fusion technologies are comprehensively reviewed. The applications of UV-Vis based methods in multi-parameter water-quality monitoring, water-quality classification, and water-quality alarm are also discussed. Finally, future research directions are outlined.