A method for preparing a nitrate ester is provided. The method includes providing a first solution including a compound (which has at least one hydroxyl group) and a carboxylic acid having 2-5 carbon atoms; providing a second solution including nitric acid, acetic anhydride, and acetic acid; and transferring the first solution and the second solution to a microreactor, obtaining a nitrate ester after a residence time. In particular, the ratio of the weight of nitric acid to the total volume of the acetic anhydride and acetic acid is 1:1 to 1:3.5. The ratio of the molar amount of nitric acid to the hydroxyl group equivalent of the compound is from 1:1 to 15:1.