A data storage device includes a flash memory and a controller. The flash memory has a plurality of SLC-spare blocks, a plurality of SLC-data blocks, a plurality of TLC-spare blocks and a plurality of TLC-data blocks. The controller writes data into the SLC-spare blocks in response to a write command arranged to write the data into the flash memory, wherein when a first predetermined condition is satisfied, the controller selects a plurality of first TLC-data blocks with the least amount of valid data from the TLC-data blocks, writes valid data stored in the first TLC-data blocks into at least one first TLC-spare block, and releases the TLC-data blocks to increase the number of TLC-spare blocks by one.