The invention discloses a preparation method of instant oat flour and relates to a preparation method of oat flour. The preparation method comprises the following steps: baking, cleaning, soaking, steaming and cooling oats, serving as raw materials, sequentially; glue milling the oats after pretreatment to prepare oat pulp, carrying out enzymatic hydrolysis, heating, boiling, and killing enzyme; and cooling an enzymolysis liquid, glue milling, homogenizing, drying, sterilizing and packaging a finished product. According to the preparation method disclosed by the invention, with the oats as the raw materials, by virtue of pretreatment, enzymatic hydrolysis technological treatment and spray drying, the instant oat flour which can keep multiple nutritional components and fragrance of oat is prepared; the prepared instant oat flour is moderate in taste, has self flavor of oat, keeps the soluble nutritional components in the oats, is also capable of providing extra plant proteins, fats and soluble dietary fibers and can be directly used for preparing an oat beverage; as macromolecular substances including starch, cellulose and proteins in the oats are hydrolyzed by virtue of an enzyme method, the viscosity of the starch of the oats is lowered, and the problems of water separating, gelation, precipitation and the like caused when the oats are applied to beverages are solved.