A thin-film transistor and a manufacturing method thereof are provided, and the manufacturing method includes: forming a source electrode, a drain electrode and a planarization layer on a substrate, and patterning the planarization layer to form a first portion disposed between the source electrode and the drain electrode, a second portion disposed at a side of the source drain, and a third portion disposed at a side of the drain electrode. Upper surfaces of all the first portion, the second portion, and the third surface are flush with top portions of both the source electrode and the drain electrode.