This invention provides an array substrate, a method for fabricating the same, and an OLED display device. Each pixel unit of the array substrate comprises: a TFT drive layer; an OLED further away from the substrate than the TFT drive layer and driven by it, the OLED sequentially comprises a first electrode, a light emitting layer, a second electrode, wherein the first electrode is transparent, and the second electrode is a transflective layer, or the second electrode is transparent and has a transflective layer disposed thereon; a reflection layer disposed between the TFT drive layer and the OLED and forming a microcavity structure with the transflective layer, and a reflective surface of the reflection layer has a concave-convex or corrugated structure disposed thereon for causing diffuse reflection of light; and a color filter film disposed between the reflection layer and the OLED and located in the microcavity structure.