The invention discloses a shield gate trench MOSFET manufacturing method. Gate structure forming comprises steps: a hard mask layer is formed, and a gate forming area is defined; a semiconductor substrate is etched to form a deep trench; thermal oxidation is carried out to form a bottom oxidation layer; source polysilicon is formed; back etching of the polysilicon is carried out, and thus, the source polysilicon is flush with the top surface of the hard mask layer, a source polysilicon lateral extension part is formed outside the opening of the bottom oxidation layer and the top part of the bottom oxidation layer inside the opening of the hard mask layer; the hard mask layer is removed to form a top convex structure of the source polysilicon; the source polysilicon lateral extension part serves as a self-alignment mask to etch the bottom oxidation layer to form top trenches and inter-polysilicon isolation oxidation layers at two side surfaces of the source polysilicon; a gate dielectric layer is formed on the side surface of the top trench; and the top trench is filled to form a polysilicon gate, and chemical mechanical polishing is carried out. Thus, while the threshold voltage of the device is reduced, the gate source electric leakage of the device is reduced.