A method for forming a three-dimensional memory device (100, 800), comprising: forming a cut structure in a stack structure, the stack structure comprising a plurality of interleaved initial sacrificial layers and initial insulating layers, forming a cut structure including forming a cut opening by anisotropic etching to remove a portion of the stack structure and filling the cut opening with a dielectric material to form the cut structure; removing portions of the stack structure adjacent to the cut structure to form a slit structure and an initial support structure, the initial support structure dividing the slit structure into a plurality of slit openings, wherein the initial support structure being formed by the cut structure and the underneath interleaved sacrificial portions and insulating portions; replacing the initial sacrificial portions with a plurality of conductor portions through the plurality of slit openings to form the initial support structure into a support structure; forming a source contact along a first direction in each of the plurality of slit openings; forming a pair of first portions of a connection layer in contact with and conductively connected to source contacts in adjacent ones of the plurality of slit openings along the first direction; forming a second portion of the connection layer in contact with and conductively to the pair of first portions of the connection layer, forming a plurality of channel structures extending vertically in a memory stack over a substrate, the memory stack comprising a plurality of interleaved conductor layers and insulating layers; wherein the initial support structure and support structure are in contact with adjacent memory blocks, rendering the memory stack and channel structures of the three-dimensional memory device less susceptible to deformation during the fabrication process. forming a cut structure in a stack structure, the stack structure comprising a plurality of interleaved initial sacrificial layers and initial insulating layers, forming a cut structure including forming a cut opening by anisotropic etching to remove a portion of the stack structure and filling the cut opening with a dielectric material to form the cut structure; removing portions of the stack structure adjacent to the cut structure to form a slit structure and an initial support structure, the initial support structure dividing the slit structure into a plurality of slit openings, wherein the initial support structure being formed by the cut structure and the underneath interleaved sacrificial portions and insulating portions; replacing the initial sacrificial portions with a plurality of conductor portions through the plurality of slit openings to form the initial support structure into a support structure; forming a source contact along a first direction in each of the plurality of slit openings; forming a pair of first portions of a connection layer in contact with and conductively connected to source contacts in adjacent ones of the plurality of slit openings along the first direction; forming a second portion of the connection layer in contact with and conductively to the pair of first portions of the connection layer, forming a plurality of channel structures extending vertically in a memory stack over a substrate, the memory stack comprising a plurality of interleaved conductor layers and insulating layers; wherein the initial support structure and support structure are in contact with adjacent memory blocks, rendering the memory stack and channel structures of the three-dimensional memory device less susceptible to deformation during the fabrication process.