The invention discloses a composition which is formed by combining plant materials and has functions of nourishing yin and reducing blood glucose, and a preparation method of the composition. The blood-glucose-reducing composition comprises cortex lycii radicis, fruit of Chinese wolfberry, glossy privet fruit, radix polygonati officinalis, Chinese yam, radix ophiopogonis and semen cassia; the yin-nourishing and blood glucose-reducing composition disclosed by the invention can be prepared into any normal internally-taken dosage form, is remarkable in curative effect for treating and preventing diabetes mellitus, and has total effective rate of over 90% for diabetes mellitus; moreover, after drug withdrawal, the impaired fasting glucose or blood glucose does not rebound two hours after the meal. The yin-nourishing and blood-glucose-reducing composition disclosed by the invention is small in side effect, has effects of tonifying spleen and reinforcing the kidneys, nourishing yin and tonifying yang, mediating internal organs and the like, treats both symptoms and root causes; after entering the body, the drug can promote islet cells to reproduce, can strengthen secretion of insulin, can improve glucose metabolism in the body, and has good prevention and treatment effect on various syndromes caused by diabetes.