A method for fabricating a semiconductor device comprises: Firstly, a semiconductor fin comprising a first sub-fin and a second sub-fin protruding from a surface of a substrate is provided. An isolation structure having an opening extending therein is then provided in the semiconductor fin to electrically isolate the first sub-fin and the second sub-fin. Subsequently, a first dummy structure disposed on the first isolation structure and having at least one metal layer entirely overlapping on the first isolation structure along a long axis of the semiconductor fin is formed, wherein the metal layer laterally conformally extends downwards into the opening and extends upwards beyond the first isolation structure along the long axis of the semiconductor fin, so as to form a stepped structure overlapping on sidewalls and a bottom of the opening, a portion of the first sub-fin and a portion of the second sub-fin.