The molecular structures of three active components from scutellaria baicalensis have been studied by ultraviolet-visible spectra. The index flavonoid structures and substituted positions were deduced by analyzing the UV-Vis spectra in the methanol solution of three active components and the methanol solution with 5 diagnostic reagents, NaOMe, NaOAc, NaOAc/H3BO3, AlCl3, AlCl3/HCl respectively, which provided strong evidences for the structural characterization of the active components from natural products.