In this letter, we have investigated the down-coupling phenomenon (DCP) of inhibited channel potential in 3D NAND flash memory. The inhibited channel in the 3D NAND flash structure can easily be in the floating state, because its channel is not directly connected to its body. This floating channel behavior can adversely affect memory operation. Specifically, during program and verify operation, the DCP reduces the boosting potential of the inhibit string. Consequently, the insufficient channel potential leads to a large program disturbance and deterioration of the cell-distribution characteristics. Using computer-aided design simulations, we analyzed this phenomenon and proposed an operation method to alleviate this phenomenon.