A method for forming a gate structure of a 3D memory device is provided. The method comprises: forming multiple hybrid shallow trench isolation structures in a substrate; forming an alternating dielectric stack on the substrate, the alternating dielectric stack including multiple dielectric layer pairs each comprising a first dielectric layer and a second dielectric layer different from the first dielectric layer; forming multiple channel structures in the alternating dielectric stack; forming a slit penetrating vertically through the alternating dielectric stack and extending in a horizontal direction to divide the multiple channel structures and to expose a row of hybrid shallow trench isolation structures; replacing the second dielectric layers in the alternating dielectric stack with multiple gate structures through the slit; forming a spacer wall to fill the slit; and forming multiple array common source contacts each in electric contact with a corresponding hybrid shallow trench isolation structure.