A method for preparing a semiconductor device, including providing a substrate, where a word line structure is formed in the substrate; a bit line supporting layer includes a first oxide layer and a first nitride layer. A bit line structure is formed in the first nitride layer, and the first oxide layer is formed on both sides of the bit line structure and located in the first nitride layer; patterning the supporting structure to form a first via corresponding to the bit line structure; and etching the bit line supporting layer to a preset height along the first via, adjusting an etching parameter and a selective etching ratio of etching gas for an oxide layer to a nitride layer, and continuing to etch the bit line supporting layer until the bit line structure is exposed, to form a polymer layer above the bit line structure.