A data storage device is disclosed. In one embodiment, the data storage device comprises a flash memory and a controller. The flash memory comprises a plurality of blocks, wherein each block comprises a plurality of pages, and each page comprises a plurality of data trimming units which is a smallest unit for data modification. After a data trimming process has been performed on an address range of the flash memory, the controller determines a last page corresponding to an ending address of the address range, determines whether data values stored in the last page with addresses subsequent to the ending address are all equal to a specific data pattern, and sets the value of a trimming flag corresponding to the last page to be 1 when the data values stored in the last page with addresses subsequent to the ending address are all equal to the specific data pattern.