The invention relates to a preparation method of a semiconductor device. The preparation method comprises the following steps of providing a semiconductor substrate which comprises a high-voltage device region and a low-voltage device region; forming a first gate oxide layer and a second gate oxide layer, wherein the thickness of the first gate oxide layer is larger than that of the second gate oxide layer; respectively forming a first polysilicon gate and a second polysilicon gate on the surface of the first gate oxide layer and the surface of the second gate oxide layer; etching the first gate oxide layer and the second gate oxide layer through using the first polysilicon gate and the second polysilicon gate as masks until the thickness of the first gate oxide layer reduced to a target thickness through etching; forming a sidewall structure and performing source-drain ion implantation for forming a source electrode leading-out region and drain electrode leading-out region. According to the preparation method of the semiconductor device, a single photoetching process is not required for reducing the thickness of the first gate oxide layer in the high-voltage device region in the etching process, thereby simplifying the process, saving a photomask layer and reducing process cost.