The present disclosure provides a 3D memory device such as a 3D AND flash memory and a method of forming a seal structure. The 3D memory device includes a chip region including a chip array and a seal region including a seal structure. The seal structure includes a ring-shaped stack structure disposed on a substrate and surrounding the chip array and a dummy channel pillar array penetrating through the ring-shaped stack structure and including a first dummy channel pillar group and a second dummy channel pillar group. The first dummy channel pillar group includes first dummy pillars that are arranged in a first direction and a second direction crossing the first direction to surround the chip array. The second dummy channel pillar group includes second dummy pillars that are arranged in the first direction and the second direction to surround the chip array. The first and the second dummy channel pillars are staggered with each other in the first and second directions.