The invention provides a method for preparing medium molecular weight hydroxyethyl starch. The method comprises the following steps of: 1) hydrolyzing amylopectin, wherein the hydrolysis degree is controlled by using a colorimetric method so that the molecular weight of the hydrolysate reaches the required range; and 2) performing etherification reaction on the hydrolysate obtained in the 1). The method for preparing the medium molecular weight hydroxyethyl starch has the advantages that: the product is high in yield and stable in quality, and the method is convenient and fast and is suitable for industrialized production.