The present invention provides a data storage device including a flash memory and a controller. The controller is configured to perform a first read operation to read a first page corresponding to a first word line of the flash memory according to a read command of a host, and perform a distribution-adjustment procedure when data read by the first read operation cannot be recovered by coding, wherein the controller is further configured to perform an adjustable read operation to read a second page corresponding to a second word line of the flash memory in the distribution-adjustment procedure.