The invention provides a method for forming a semiconductor structure. The method includes providing a substrate, forming a gate structure on the substrate, respectively forming an epitaxial layer on both sides of the gate structure, and performing a pre-amorphization doping step on the substrate. After the pre-amorphization doping step, a defect is generated in the epitaxial layer, an outer spacer is formed beside the gate structure, and a chemical cleaning step is performed to remove a part of the epitaxial layer, and the defect in the epitaxial layer is removed.