An oxide thin film transistor, an array substrate, and preparation methods thereof are disclosed. The method for preparing an oxide thin film transistor comprises a step of forming a pattern comprising an oxide semiconductor active layer on a substrate, wherein the step comprises: forming an amorphous oxide semiconductor thin film on the substrate; performing an excimer laser annealing, at least at a position in the amorphous oxide semiconductor thin film corresponding to a channel region of oxide semiconductor active layer to be formed, such that the amorphous oxide semiconductor material at the laser-annealed position is crystallized, to form a crystalline oxide semiconductor material; and forming the pattern comprising the oxide semiconductor active layer.