NOVELTY - Producing quercetin involves adding inorganic strong acid in water to obtain acid-water mixture, then heating to boiling and adding rutin, where the rutin feeding quantity is (per kg) water mixture (7-15 l) and acid-water mixture (0.4-1 mol/l); keeping the boiling for at least 1 hour; filtering the hydrolyzed solution and collecting the filtered solution; and washing obtained filter cake with water to wash out the liquid neutral pH, drying to obtain quercetin crude product filter cake, refining the cake, and washing with water to colorless and flavorless, and drying to obtain quercetin.    USE - For producing quercetin (claimed) used for treating diabetes mellitus complication, ischemia and arrhythmia; for reducing blood pressure, blood capillary, capillary blood fat, reducing blood fat; for expanding coronary artery, increasing coronary; and as antibacterial, antiviral, and anti-inflammatory agent.    ADVANTAGE - The method is environment friendly, can be performed without any other emissions, and is an old process without pollution; is simple, easy to control, has high conversion rate, and greatly reduces the cost; and provides high product purity.    DETAILED DESCRIPTION - Producing quercetin involves adding inorganic strong acid in water to obtain acid-water mixture, then heating to boiling and adding rutin, where the rutin feeding quantity is (per kg) water mixture (7-15 l), and acid-water mixture (0.4-1 mol/l); keeping the boiling for at least 1 hour; filtering the hydrolyzed solution and collecting the filtered solution; and washing the obtained filter cake at a temperature of 60-70 degrees C with water to wash out the liquid neutral pH, where washing is neutral stopping washing, drying to obtain quercetin crude product filter cake, refining the quercetin crude product filter cake into 5-15 times fine methanol to form quercetin rough filter cake, stirring uniformly, standing, filtering the filter cake with 10 times fine methanol and washing with 2-10 times pure water to colorless and flavorless, and drying to obtain quercetin.