Quercetin is widely present in the stem skin, flowers, leaves, buds, seeds and fruits of many plants, mostly in the form of glycosides, such as rutin, quercetin, hypericin, etc. Quercetin can be obtained by acid hydrolysis. Among them, the content is higher in the stem and leaves of buckwheat, sea buckthorn, hawthorn and onion. Quercetin is also found in many foods, including Onions, shallots, asparagus, cabbage, mustard greens, green peppers, chicory, grapefruit, lettuce, hawthorn, apples, mangoes, plums, radishes, blackcurrant, potatoes, and spinach. In addition, about 100 kinds of medicinal plants (such as sophora rice, platycypress leaves, galangal, colttoes, mulberry parasite, Notoginseng, ginkgo biloba, elderberry, etc.) contain this ingredient.