A bottom gate thin film transistor and an active array substrate are provided. The bottom gate thin film transistor includes a gate, a gate insulation layer, a semiconductor layer, a plurality of sources and a plurality of drains. The gate insulation layer is disposed on the gate. The semiconductor layer is disposed on the gate insulation layer and located above the gate. An area ratio of the semiconductor layer and the gate is about 0.001 to 0.9. The sources are electrically connected with each other, and the drains are electrically connected with each other.