An organic thin film transistor and a preparation method thereof, an array substrate and a preparation method thereof, and a display device; and the preparation method of the organic thin film transistor comprises: forming a source-drain metal layer including a source electrode (12a) and a drain electrode (12b), and forming an organic semiconductor active layer (13) in contact with the source electrode (12a) and the drain electrode (12b); and forming an organic insulating thin film (140) on a substrate (10) where the source-drain metal layer and the organic semiconductor active layer (13) have been formed, thinning the organic insulating thin film (140) and curing the thinned organic insulating thin film (140), or curing the organic insulating thin film (140) and thinning the cured organic insulating thin film (140), to form an organic insulating layer (14). The method can be used to form a thin and uniform organic insulating layer, so a technical difficulty in forming a via hole is reduced.