The extraction of rutin from Sophora japonica L. flower buds by ultrasonic assistance technology was investigated, and the optimum conditions were obtained as follows: ultrasonic frequency and power were 20 kHz and 800 W, ratio of material to fluid was 1∶15, and the extraction was carried out at 20 ℃ for 10 min and repeated twice. The extracted rutin was further purified by the method of changing molecular formation, and its purity was determined by HPLC. With AAS, UV spectra and IR spectra, the structure of the complex of rutin with chromium (Cr3+) were studied. The results showed that the molar ratio of rutin to chromium (Cr3+) was 1∶2. Meanwhile, 5-hydroxyl group, 4-carbonyl group and ortho-dihydroxyphenyl in ring B were found to have taken part in the coordination reaction with metal ions Cr3+.