The invention provides a preparation method of a flaky nanometer zinc oxide powder, which adopts the following preparation steps: (1) preparing a water-soluble zinc salt solution; (2) under mixing in the room temperature, dripping ammonia water or ammonium salt in the solution till the pH=11, then adding PEG with 0.1-0.6% weight of solute, and finally transferring the prepared solution into a reaction vessel; (3) conducting hydro-thermal treatment of the material in the reaction vessel under the closed condition with the treatment temperature of 160-240 DEG C and the time of 3-24 hours; and (4) cleaning and drying the product with the deionized water so as to obtain the white flaky nanometer zinc oxide powder. The single dispersed nanometer zinc oxide powder can be directly obtained without roasting and grinding for the synthesis of powders, thereby simplifying the production process for industrialized production, reducing the equipment investment, lowering the production cost and being beneficial to mass production.