Embodiments of bonded 3D memory devices and fabrication methods thereof are disclosed. In an example, a 3D memory device includes a first semiconductor structure and a second semiconductor structure. The first semiconductor structure includes a plurality of first NAND memory strings and a plurality of first BLs. At least one of the first BLs may be conductively connected to a respective one of the first NAND memory strings. The first semiconductor structure also includes a plurality of first conductor layers, and a first bonding layer having a plurality of first bit line bonding contacts conductively connected to the plurality of first BLs and a plurality of first word line bonding contacts conductively connected to the first conductor layers. A second semiconductor structure includes a plurality of second NAND memory strings and a plurality of second BLs.