A method for manufacturing a buried word line transistor can include the following operations. A semiconductor substrate having an active region is provided. A first trench is formed in the active region. A first insulation layer is formed on a side wall of the first trench. A bottom portion of the first trench is etched to form a second trench. A gate oxide layer is formed on a side wall of the first insulation layer and a bottom portion and a side wall of the second trench. A barrier layer is formed at a bottom portion and portion of a side wall of the gate oxide layer. A metal filler layer is formed on an inner side of the barrier layer. The first insulation layer is removed to form a side trench. A second insulation layer is formed at a top end of the side trench. A sealed air spacer layer is formed.