The present disclosure relates to a thin film transistor and a manufacturing method thereof. The thin film transistor includes a substrate, a first semiconductor layer, a gate dielectric layer, and a gate electrode sequentially stacked on the substrate, the first semiconductor layer has a first portion located in a channel region of the thin film transistor and a second portion in source/drain regions of the thin film transistor and located on both sides of the first portion, the second portion and first sub-portions of the first portion adjacent to the second portion include an amorphous semiconductor material, a second sub-portion of the first portion between the first sub-portions includes a polycrystalline semiconductor material, and a second semiconductor layer located in the source/drain regions and in contact with the second portion, wherein a conductivity of the second semiconductor layer is higher than a conductivity of the amorphous semiconductor material.