The invention provides a method for preparing a ZnO thick film. The method comprises the following steps: dispersing flaky basic zinc salt grains into water to form precursor slurry; filtering the precursor slurry by a microporous filtering film to make the flaky basic zinc salt grains autogenously assembled into an orientation basic zinc salt thick film; transferring the basic zinc salt thick film on the microporous filtering film onto a substrate; and heating and decomposing the thick film to obtain the ZnO thick film. The method uses the flaky basic zinc salt as the precursor to prepare the ZnO thick film, and the prepared ZnO thick film has the advantages of good orientation, even thickness and stable structure.