The present invention provides a data storage device including a flash memory and a controller. The flash memory is capable of operating in a SLC mode and a non-SLC mode. The controller is configured to perform a first read operation to read a page corresponding to a first word line of the flash memory in the SLC mode according to a read command of a host, and perform an adjustable read operation when data read by the first read operation cannot be recovered by coding, wherein the controller is further configured to enable the flash memory to operate in the non-SLC mode in the adjustable read operation, and write logic 1 into a most-significant-bit page corresponding to the first word line in the non-SLC mode to adjust voltage distribution of the first page.