Provided is a preparation method of a semiconductor device, including the following steps: providing a substrate and forming a mask layer with a plurality of first windows on the substrate; forming a dielectric layer, the dielectric layer at least covering sidewalls of the first windows; forming a first photoresist material layer, the first photoresist material layer covering the dielectric layer and the mask layer and filling the first windows; patterning the first photoresist material layer to form a patterned first photoresist layer which exposes a top surface of the dielectric layer; by using the first photoresist layer and the mask layer as masks, removing the dielectric layer to form second windows; and removing part of the substrate along the second windows to form a patterned substrate.