The invention provides a preparation method of a top gate metal oxide thin film transistor (TFT), belonging to the technical field of semiconductors. The method comprises the following steps: preparing a source/drain electrode; then preparing a metal oxide material; coating a photoresist on the surface of the metal oxide material and carrying out chemical mechanical polishing on the photoresist layer; processing the metal oxide which is not masked by the photoresist by annealing or plasmas; and finally stripping the photoresist and depositing a gate insulation layer material through magnetronsputtering and forming a gate through wet etching. The method has the following beneficial effects: reliable ohmic contact between the metal oxide thin film and the source/drain electrode can be formed by utilizing the characteristic that the metal oxide material can be converted into a conductor from a semiconductor after undergoing special processing.