The disclosure discloses a preparation method of slowly digestible starch, and belongs to the technical field of starch modification. In the disclosure, ordinary rice starch and corn starch are used as raw materials, and the starch is fully de-clustered through dry-heat amorphization treatment, instead of traditional hydrothermal gelatinization; then ultrasonic treatment is used to induce complexation of amino acids and starch milk to form VI-type crystal; and finally through an extrusion technology, the VI-type crystal is transformed into VII-type crystal, and the content of the slowly digestible starch is significantly increased to 55%-60%. The disclosure is highly efficient, simple and environmentally friendly, and the prepared product can be used as a drug sustained-release carrier or a low-glycemic index food ingredient, and has important practical value for the regulation of chronic diseases such as diabetes and cardiovascular diseases.