The present invention provides a thin-film transistor and a manufacturing method thereof. The manufacturing method of the thin-film transistor according to the present invention is such that by forming a first photoresist layer on an active layer and using a mask associated with the active layer to pattern the first photoresist layer so as to form the first photoresist pattern, the first photoresist pattern so formed provides protection of the active layer against corrosion caused by acidic etchant solution in the subsequently conducted etching operation of source and drain electrodes so as to function as an etching stopper layer and further, a major portion of the first photoresist pattern can be removed in a photolithographic process of the source and drain electrodes so that only a minor portion is left in the finally-formed thin-film transistor and does not affect the properties of the thin-film transistor. The thin-film transistor according to the present invention has a simple manufacturing process and a low manufacturing cost and the surface of the active layer is flat and smooth. The thin-film transistor shows excellent properties.