The present invention provides an array substrate and a manufacturing method thereof and a display device. The manufacturing method comprises: forming a pattern including a pixel electrode and a source of a thin film transistor on a base substrate through a single patterning process, the pixel electrode is provided in a layer under a layer in which the source is located; forming a pattern including a drain, an active layer, a gate insulation layer and a gate of the thin film transistor through a single patterning process, the active layer covers the source and the drain, and is separated from the gate through the gate insulation layer; and forming a pattern including a passivation layer, a common electrode and a gate line through a single patterning process, the common electrode is a slit electrode and separated from the active layer and the pixel electrode through the passivation layer.