A control method for a memory device is provided. The control method includes the following steps. Convert multiple input bits on multiple bit-channels into a code word through a polar code transformation. Select a boundary bit-channel among the bit-channels according to a first ranking list for the bit-channels. Identify a target memory cell among the memory cells according to the boundary bit-channel and a generator matrix of the polar code transformation. Decrease a raw bit error rate of the target memory cell.