Joint opening structures of 3D memory devices and fabricating method are provided. A joint opening structure comprises a first through hole penetrating a first stacked layer and a first insulating connection layer, a first channel structure at the bottom of the first through hole, a first functional layer on the sidewall of the first through hole, a second channel structure on the sidewall of the first functional layer, a third channel structure over the first through hole, a second stacked layer on the third channel structure, a second insulating connection layer on the second stacked layer, a second through hole penetrating the second stacked layer and the second insulating connection layer, a second functional layer disposed on the sidewall of the second through hole, a fourth channel structure on the sidewall of the second functional layer, and a fifth channel structure over the second through hole.