The invention discloses an array substrate and a preparation method thereof. The array substrate comprises a substrate, a buffer layer, a semiconductor pattern layer, a first grid electrode insulation layer, a second grid electrode insulation layer, a first conductive pattern layer, an interlayer insulation layer and a second conductive pattern layer, wherein the substrate comprises a thin-film transistor region and a capacitor region; the buffer layer is formed on the substrate; the semiconductor pattern layer is formed on the buffer layer and comprises a source and drain electrode region and a first electrode; the first grid electrode insulation layer is formed on the semiconductor pattern layer and correspondingly covers regions, except the first electrode, of the thin-film transistor region and the capacitor region; the second grid electrode insulation layer is formed on the first grid electrode insulation layer; the first conductive pattern layer is formed on the second grid electrode insulation layer and comprises a grid electrode and a second electrode; the interlayer insulation layer is formed on the first conductive pattern layer; the second conductive pattern layer is formed on the interlayer insulation layer and comprises a source electrode and a drain electrode which are coupled with the source and drain electrode region as well as a third electrode. With the adoption of the array substrate and the preparation method thereof, the capacitance of a capacitor is increased, film layer structures of the regions except the capacitor region are not changed, the signal line load is effectively decreased, and meanwhile, short-circuit risks of upper and lower film layers of the grid electrode insulation layers are reduced.