A data storage device includes a flash memory, a data processing module and a flash memory controller. Corresponding to the operation of a host, the flash memory controller arranges the flash memory to store data, and it stores a mapping table to record the mapping information between the flash memory and the logical address of the host. When the host transmits a trim command to invalidate a specific portion of the mapping table and the host manages to read the data of the specific portion, the flash memory controller sets up a flag to be open so that the data is transmitted to the host without the implement of the data processing module.