Photochemical degradation of chromophoric-dissolved organic matter (CDOM) by UV-B radiation decreases CDOM absorption in the UV region and fluorescence intensity, and alters CDOM composition. CDOM absorption, fluorescence, and the spectral slope indicating the CDOM composition were studied using 0.22-μm-filtered samples of Meiliang Bay water from Lake Taihu that were exposed to short-term (0–12 h) simulated UV-B radiation and long-term (0–12 days) natural solar radiation in summer. CDOM absorption coefficient and fluorescence decreased with increasing exposure time, which relates to the amounts of absorbed light energy. The decreases of CDOM absorption and normalized fluorescence corresponded to first order kinetics reactions. Different decreases of CDOM absorption and fluorescence at different wavelengths suggested that the composition of CDOM changed when it absorbed ultraviolet radiation. Photochemical degradation increased the spectral slope during 275–295 nm region (S 275–295) but decreased the spectral slope during 275–295 nm region (S 350–400). The slope ratio S R (S 275–295:S 350–400) increased in the photochemical process, which could be used as an indicator of photobleaching and composition change of CDOM. Our results show that photochemical degradation is important in the cycling of CDOM, which indicated change in the composition of CDOM.