An array substrate, a manufacturing method thereof and an OLED display device are provided. The array substrate comprises a plurality of pixel units disposed on a substrate, wherein each pixel unit includes a TFT structure formed on the substrate and an OLED driven by the TFT structure; the TFT structure includes a drive TFT. A drain electrode of the drive TFT is connected with the OLED; a gate electrode and the drain electrode of the drive TFT are at least partially overlapped to form a storage capacitor. A recess is formed in an insulating layer interposed between the gate electrode and the drain electrode corresponding to the overlapped area, so that the distance between the gate electrode and the drain electrode is less than the thickness of the insulating layer corresponding to the non-overlapped area.