A method for forming a 3D memory device is provided. The method comprises forming a sacrificial layer on a substrate, forming an alternating dielectric stack on the sacrificial layer, forming a plurality of channel holes vertically penetrating the alternating dielectric stack and the sacrificial layer, and forming a first channel layer in each channel hole. The method further comprises forming a second channel layer on the first channel layer in each channel hole, such that a merging point of the second channel layer is higher than a bottom surface of the alternating dielectric stack. The method further comprises removing the sacrificial layer to form a horizontal trench, and forming a selective epitaxial growth layer in the horizontal trench.