The invention discloses a preparation method of selenium-enriched nutritious instant coarse cereal powder. The preparation method comprises the steps of precooking coarse cereal raw materials with microwave, performing crushing and screening, fully and uniformly mixing the screened material with rice powder, millet powder, buckwheat powder, taro powder, Chinese yam powder, sesame powder, highlandbarley powder, saffron, maltodextrin, milk powder and a selenium diluent, and adopting an extrusion puffing technology to obtain the selenium-enriched nutritious instant coarse cereal powder. The instant powder is comprehensively balanced, good in brewing effect, and strong in instant solubility, and has no caking, no precipitation at the bottom, uniform and delicate texture, good taste and delicious color. The method improves the defects of traditional coarse cereal powder, such as inconsistent cooking time, rough taste, poor solubility, poor taste and single nutrition. The selenium-enrichednutritious instant coarse cereal powder prepared by the method has increased soluble selenium content, improves the digestibility and absorptivity of selenium by the human body, meets the needs of modern fast-paced life for healthy diet, enriches the variety of coarse cereals and expands the application scope of coarse cereals.