The present invention relates to flaky alpha-alumina crystals with large aspect ratio and its preparation method, and particularly to flaky alpha-alumina crystals comprising aluminum oxide, zinc oxide and tin oxide, which are prepared by obtaining a mixed gel which are prepared by obtaining a mixed gel by means of hydrolysis of an aqueous aluminum precursor solution comprising aqueous flux, aqueous zinc precursor solution and tin precursor aqueous solution, followed by aging, drying and crystallization processes. The crystals herein have an average particle thickness of 0.5 μm or less, an average particle diameter of 30 μm or higher and an aspect ratio of 100 or higher, thus being useful as a substrate of high quality pearlescent pigments, an abrasive, ceramic material and a filling material.