An embodiment of the present disclosure provides a method for forming a gate structure, comprising: providing a substrate, where the substrate includes a nMOSFET area and a pMOSFET area, each of the nMOSFET area and the pMOSFET area has a gate trench, and each of the gate trenches is provided at a bottom portion with a gate dielectric layer; forming a gate dielectric capping layer on a surface of the substrate; forming an oxygen scavenging element layer on the gate dielectric capping layer; forming an etching stop layer on the oxygen scavenging element layer; forming a work function adjustment layer on the etching stop layer; performing metal layer deposition and annealing process to fill the gate trenches with a metal layer; and removing the metal layer outside the gate trenches.