The structure changes of flavonoids in strong acid and alkaline solution can be characterized by means of ultraviolet-visible (UV-Vis) spectrum. In this paper, the shifts of UV-Vis spectrum of quercetin in aqueous ethanol solution with different pH values were investigated. The UV-Vis spectrum of quercetin in 50% (v/v) aqueous ethanol solution showed two characteristic adsorption bands setting at 374 nm and 256 nm, respectively. When the pH value was adjusted to 14.0 with sodium hydroxide, the band at 374 nm shifted to about 400 nm, and the band at 256 nm disappeared, a new band at 314 nm appeared. The pH value of alkaline solution was adjusted to 7.0 with hydrochloric acid, and the UV-Vis spectrum of precipitate was the same with quercetin, but the UV-Vis spectrum of filtrate was different. When the pH value of quercetin in 50% (v/v) aqueous ethanol solution was adjusted to 1.0 with hydrochloric acid, there was little shift of UV-Vis spectrum. But after concentrating the acid solution and filtering, the UV-Vis spectrum of precipitation was the same with quercetin, while the UV-Vis spectrum of filtrate was different. It was concluded that strong acid and alkaline caused some degradation of quercetin.