The invention relates to a FinFET (Fin Field Effect Transistor) and a formation method thereof. The formation method of the FinFET comprises the steps of providing a substrate, wherein the substrate comprises a first semiconductor layer, an insulating layer and a second semiconductor layer, forming a mask layer at partial surface of the second semiconductor layer and etching the second semiconductor layer by taking the mask layer as a mask to the insulating layer so as to form a fin portion, forming a sacrificial layer at the surface of the sacrificial layer, wherein the surface of the sacrificial layer is lower than top surface of the fin portion and covers partial side wall of the fin portion, and the part, which is higher than the sacrificial layer, of the fin portion acts as a first sub-fin portion, forming a protection layer at the side wall surface of the first sub-fin portion, removing the sacrificial layer, carrying out transverse etching on the fin portion along the exposed side wall of the fin portion so as to enable the width of partial fin portion to be reduced and enable the first sub-fin portion to be partially suspended, wherein the part, which is reduced in width, of the fin portion acts as a second sub-fin portion, and removing the protection layer and the mask layer and forming a gate structure which stretches across the first sub-fin portion and the second sub-fin portion. The formation method provided by the invention can improve the performance of the formed FinFET.