One or more embodiments of the disclosed technology provide a thin film transistor, an array substrate and a method for preparing the same. The thin film transistor comprises a base substrate, and a gate electrode, a gate insulating layer, an active layer, an ohmic contact layer, a source electrode, a drain electrode and a passivation layer prepared on the base substrate in this order. The active layer is formed of microcrystalline silicon, and the active layer comprises an active layer lower portion and an active layer upper portion, and the active layer lower portion is microcrystalline silicon obtained by using hydrogen plasma to treat at least two layers of amorphous silicon thin film prepared in a layer-by-layer manner.