A thin film transistor and a preparation method thereof and an array substrate are provided, the thin film transistor includes a substrate, a gate, a gate insulating layer, a first metal layer, an active layer, a second metal layer, and a passivation layer, the gate is disposed on the substrate, the gate insulating layer covers the gate, the first metal layer includes a first electrode and a second electrode spaced apart on the gate insulating layer, the second metal layer includes a source and a drain spaced apart, one end of the active layer is sandwiched between the first electrode and thesource electrode, the other end of the active layer is sandwiched between the second electrode and the drain electrode, and the passivation layer covers the exposed surfaces of the source electrode, the drain electrode and the active layer. By sandwiching the active layer between the first metal layer and the second metal layer, the contact performance between the active layer and the second metallayer is improved. In the preparation method of the present invention, the first metal layer and the second metal layer are formed by two times ofa composition process, thereby avoiding the occurrence of a chamfer.