The invention provides a fin type field effect transistor with SiGeSn source drain and a forming method thereof. The forming method comprises the following steps of providing a substrate, forming a fin-shaped Ge structure on the substrate, forming a gate stack or a pseudo gate on the fin-shaped Ge structure, forming an opening of a source area and an opening of a drain area in the two sides of the gate stack or the pseudo gate, exposing the fin-shaped Ge structure from the positions of the openings, and injecting atoms or molecules or ions or plasma containing a Si element and a Sn element into the fin-shaped Ge structure so that an SiGeSn layer can be formed at the positions of the openings. According to the forming method of the fin type field effect transistor, a FinFET with the SiGeSn source drain can be formed; the thickness of the SiGeSn source drain is small, the crystalline quality is good, and therefore the transistor has the good electrical property; the method has the advantages of being simple and easy to implement and low in cost.