The invention discloses a fin field effect transistor and a forming method thereof. The forming method comprises the following steps: after a fin part structure is formed, first grooves are formed on the surface of the top part of the fin part structure, and the fin part structure on the side wall of the first groove forms first fin parts; second grooves are formed on the surfaces of the bottom parts of the first grooves, and the fin part structure on the side walls of the second grooves forms second fin parts; and point angles of the first fin parts and the second fin parts are changed into arcs. As the surface of the top part of the fin part structure is etched to form a plurality of first fin parts and second fin parts, the surface area of the finally-formed fin part structure is far greater than the surface area of the prior fin part with a rectangular section, improvement of the groove area of the fin field effect transistor is facilitated, and improvement of a driving current is facilitated; and as the point angles of the first fin parts and the second fin parts are changed into arcs, current in partial groove areas can be prevented from being excessively big to cause the device to overheat.