A method is provided for fabricating a FinFET. The method includes providing a substrate including an NMOS region; forming a plurality of fins on the substrate; forming an isolation layer between adjacent fins and on the substrate; forming a gate structure across a length portion of the fin; forming a first mask layer on the top surface and sidewalls of the fin; etching the first mask layer to expose the top surface of the fin on both sides of the gate structure; removing a thickness portion of the fin on both sides of the gate structure, wherein the etched fin and the remaining first mask layer form a first trench; performing a thinning treatment of the remaining first mask layer on a sidewall of the first trench to increase width of the first trench; and forming an N-type in-situ doped epitaxial layer to fill up the first trench.