The present disclosure provides a method for preparing a semiconductor device with a T-shaped buried gate electrode. The method includes forming an isolation structure in a semiconductor substrate to define an active region, and forming a doped region in the active region. The method also includes etching the semiconductor substrate to form a first trench and a second trench. The first trench has a first portion extending across the doped region and a second portion extending away from the first portion, and the second trench has a third portion extending across the doped region and a fourth portion extending away from the third portion. The method further includes forming a first gate electrode in the first trench and a second gate electrode in the second trench.