Memory system and access method thereof

The invention discloses a memory system and an access method thereof. The memory system includes a plurality of memory cells, a converting circuit and an access circuit. The access method includes the following steps: providing a corresponding relationship between M threshold voltages and N data bits for each memory cell, wherein the M threshold voltages contain at least one threshold voltage having higher anti-interference ability and at least one threshold voltage having lower anti-interference ability; replacing a combination of a data bit corresponding to the at least one threshold voltage having the lower anti-interference ability in the corresponding relationship with the at least one threshold voltage having the higher anti-interference ability; and storing the memory cells according to the corresponding relationship after replacement.