The invention discloses a preparation method of a metal gate structure. The method comprises the steps of forming a silicon nitride layer above a dummy gate, forming a partial side wall covering a dummy gate structure and an auxiliary side wall with the upper surface flush with the dummy gate structure, partially etching the dummy gate structure, removing the residual dummy gate, forming a T-shaped gate groove consisting of the residual main side wall, the residual auxiliary side wall and an exposed part of a substrate, and then forming a metal gate in the T-shaped gate groove, wherein the silicon nitride layer protects the surface of the dummy gate against silicide forming in a process of forming sources/drains in the substrate by a source/drain ion implantation technology and further avoids polycrystalline silicon residue generation in a subsequent dummy gate removal technology. The top size of the T-shaped gate groove is greater than the bottom size, so that a metal gate material can be filled more sufficiently, and the performance of a subsequently formed semiconductor device is improved.