An OLED device and a preparation method thereof, and a display device are provided. The OLED device comprises a substrate and a plurality of functional layers disposed sequentially on the substrate. One functional layer of the plurality of functional layers is a transition functional layer, the transition functional layer comprises a first sub-layer and a second sub-layer provided on the first sub-layer, the first sub-layer and the second sub-layer are made of a same material; and the first sub-layer is prepared by using a first process, and the second sub-layer is prepared by using a second process different from the first process.