The present disclosure provides a method for preparing a semiconductor device with air spacer for decreasing electrical coupling. The method comprises: forming a plurality of composite pillars over a substrate, wherein the composite pillars include conductive pillars and dielectric caps over the conductive pillars; transforming a sidewall portion of the conductive pillar into a first transformed portion; removing the first transformed portion such that a width of the dielectric cap is greater than a width of a remaining portion of the conductive pillar; forming a supporting pillar between adjacent two of the plurality of composite pillars; and forming a sealing layer at least contacts a top portion of the supporting pillar and a top of the dielectric cap, and air spacers are formed between the sealing layer, the supporting pillar and the remaining portions of the conductive pillars.