A method for preparing a trench isolation structure, which comprises the following steps of: providing a substrate; forming an oxide layer on the substrate; successively generating an oxidation barrier layer and an ethyl orthosilicate layer on the surface of the oxide layer; etching the oxidation barrier layer and the ethyl orthosilicate layer; corroding the substrate to form a trench by using the oxidation barrier layer and the ethyl orthosilicate layer as mask layers; removing the ethyl orthosilicate layer, and oxidizing a side wall of the trench by using the oxidation barrier layer as a barrier layer; filling the trench with a polysilicon and then etching back the polysilicon, and removing the polysilicon on the surface of the oxidation barrier layer; and removing the oxidation barrier layer and the oxide layer on the surface of the substrate.