The invention discloses a fin field effect transistor and a forming method thereof. The forming method comprises steps: a semiconductor substrate is provided, wherein the surface of the semiconductor substrate is provided with a convex fin part; a metal sulfide layer crossing the top and the side wall of the fin part and located on the surface of the semiconductor substrate is formed, and the metal sulfide layer subsequently serves as a channel region of the fin field effect transistor; a first gate structure located on the surface of the metal sulfide layer is formed, and the first gate structure crosses the top and the side wall of the fin part; and a source electrode layer and a drain electrode layer located at two sides of the first gate structure are formed. The formed fin field effect transistor is high in carrier mobility, and the transistor performance is excellent.