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 the substrate; forming an etching stop layer on the gate dielectric capping layer; forming an oxygen scavenging element layer on the etching stop layer; forming a first work function adjustment layer on the oxygen scavenging element layer; etching the first work function adjustment layer above the nMOSFET area; forming a second work function adjustment layer on the surface of the substrate; metal layer depositing and annealing to fill the gate trenches with a metal layer; and removing the metal layer outside the gate trenches.