An organic light emitting diode (OLED) display device (01) and a fabrication method thereof are provided. The OLED display device (01) comprises a plurality of sub-pixel units, and each of the sub-pixel units includes a first electrode (20), an organic material functional layer (30) and a second electrode (40) which are sequentially disposed on a base substrate (10). The sub-pixel unit further includes: a first buffer layer (50) disposed between the base substrate (10) and the first electrode (20). A surface of a part, corresponding to the first electrode (20), of the first buffer layer (50) on a side away from the base substrate (10) is in a concave shape; the first electrode (20) is a transparent electrode, and the second electrode (40) is a non-transparent metal electrode.