The invention discloses a method for improving gate oxide breakdown voltage, and particularly provides a shallow trench gate oxide layer. A chlorine processing sacrificial oxide layer is developed after the gate oxide layer is pre-cleaned; the developed sacrificial oxide layer is removed by using an acid bath; and a gate oxide structural layer is developed on the surface of the gate oxide layer. According to the method disclosed by the invention, optimization is carried out on corners of the shallow trench gate layer through a mode of additionally arranging the chlorine processing sacrificial oxide layer and removing the chlorine processing sacrificial oxide layer, thereby achieving an effect of enabling the gate oxide corners to be smooth, and avoiding a problem that the gate oxide breakdown voltage is lowered because of the corners of the shallow trench gate oxide layer is sharp. The gate oxide thickness of a semiconductor device prepared by adopting the method disclosed by the invention achieves requirements, the corners of the shallow trench gate oxide layer are smooth, obvious sharp corners does not exist, and the breakdown voltage of the gate oxide layer is also within the normal range.