The invention provides a fin type field effect transistor with a GeSn channel 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, injecting atoms or molecules or ions or plasma containing a Sn element into the fin-shaped Ge structure so that a GeSn layer can be formed, and forming a gate stack structure on the GeSn layer. According to the forming method, a FinFET with a GeSn channel area small in thickness and good in quality can be obtained; the method has the advantages of being simple and easy to implement and low in cost.