The present invention provides a data storage device including a flash memory and a controller. The controller equally distributes the TLC-data blocks into three regions. In a first stage, the controller determines a first TLC-data block corresponding to the logic address of a prewrite data sector, defines the region that contains the first TLC-data block as a first region, and determines whether the first TLC-data block has valid data. When the first TLC-data block does not have valid data, the controller selects a second TLC-data block and a third TLC-data block from the regions other than the first region for writing the prewrite data sector, into the first TLC-data block, the second TLC-data block and the third TLC-data block by a SLC storage mode.