A MOSFET structure and a manufacturing method thereof are provided. The structure includes a substrate, a well region of a first conductivity type, a first trench formed on a surface of the well region of the first conductivity type and extending downwards to a well region of a second conductivity type, a source disposed in the well region of the second conductivity type and under the first trench, a gate oxide layer disposed on an inner surface of the first trench, a polysilicon gate disposed on the gate oxide layer, a conductive plug extending downwards from above the first trench and being in contact with the well region of the second conductivity type after extending through the source, an insulation oxide layer filled in the first trench between the conductive plug and the polysilicon gate, and a drain disposed outside the first trench and obliquely above the source.