Semiconductor memory device and forming method thereof

The invention discloses a semiconductor memory device and a forming method thereof. The semiconductor memory device comprises a substrate, a plurality of bit lines, a plurality of plugs and a gap wall structure. The bit lines are arranged on the substrate in a mutually separated manner. The plugs are disposed on the substrate and alternately disposed with the bit lines. The gap wall structure is arranged on the substrate and located between the bit line and the plug, the gap wall structure comprises a first gap layer, a first gap wall and a second gap layer, and the first gap layer, the first gap wall and the second gap layer are sequentially stacked between the side wall of the bit line and the plug. And the bottommost surface of the first gap layer and the bottommost surface of the second gap layer are not equal in height. Therefore, two gap layers can be formed between the bit line and the storage node plug, so that the condition of delay between resistance and capacitance is effectively improved. 本发明公开了半导