The invention discloses a preparation method of flaky nanometer gamma-aluminum oxide. The preparation method includes the steps that 1, inorganic aluminum salt and urea are dissolved in water to obtain a transparent solution, and the solution is transferred into a high-pressure reaction kettle; 2, hydrogen is injected into the high-pressure reaction kettle, and certain pressure and temperature are kept for a reaction; 3, after the reaction is ended, reaction materials are subjected to filtering, washing and drying to obtain a boehmite precursor, and the flaky nanometer gamma-aluminum oxide is obtained after calcining. The method has no neutralizing, ageing and other complex steps, and has the advantages of be simple in technology and having a few reaction steps. The gamma-aluminum oxide powder with a large specific surface area and the uniform flaky structure can be obtained, and is suitable for synthesis of a nanometer gamma-aluminum oxide carrier, for a metal supported catalyst, with a large specific surface area.