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In this work, waxy maize starch nanocrystals were prepared by mixed acid of sulfuric acid and hydrochloric acid for the first time. Physicochemical properties, crystalline structure, and particle size of starch nanocrystals prepared by mixed acid (HSNC) were measured. The results showed that there was no difference in particle morphology, group structure, and surface elements of HSNC and starch nanocrystals prepared by sulfuric acid (SNC), which was a conventional preparation method. The yield of HSNC was lower than that of SNC. However, the preparation time of HSNC was greatly shortened to 1 h which is only 0.83 % of the time (5 d) to prepare SNC by the sulfuric acid, and HSNC showed higher relative crystallinity and smaller size than those of SNC. In addition, when the ratio of sulfuric acid and hydrochloric acid to provide H ion concentration was 1:1 to hydrolyze starch for >1 h, the crystal pattern of HSNC would be changed to V-typed, while the crystal pattern of SNC was still A-typed. Thus, this work provided efficient method for preparing starch nanocrystals is provided, which can replace sulfuric acid hydrolysis to prepare SNC. • Mixed acid were used to prepare starch nanocrystals (HSNC) in 1 h • Starch nanocrystals prepared by mixed acid shortened than conventional preparation • HSNC prepared by mixed acid had higher relative crystallinity and smaller size • The crystal pattern of HSNC prepared >1 h would change to V-typed [ABSTRACT FROM AUTHOR]