A method for forming a gate structure of a 3D memory device is provided. The method comprises: forming, on a substrate, an alternating dielectric stack including a plurality of dielectric layer pairs, each of the plurality of dielectric layer pairs comprising a first dielectric layer and a second dielectric layer different from the first dielectric layer; forming a slit penetrating vertically through the alternating dielectric stack and extending in a horizontal direction; removing the plurality of second dielectric layers in the alternating dielectric stack through the slit to form a plurality of horizontal trenches; forming a gate structure in each of the plurality of horizontal trenches; forming a spacer layer on sidewalls of the slit to cover the gate structures, wherein the spacer layer has a laminated structure; and forming a conductive wall in the slit, wherein the conductive wall is insulated from the gate structures by the spacer layer.