A data storage device is provided. The data storage device includes: a flash memory, a dynamic random access memory (DRAM), and a memory controller. The flash memory stores a logical-to-physical mapping (L2P) table that is divided into a plurality of group-mapping tables. The memory controller receives a host command from a host, wherein the host command includes one or more pieces of data and one or more corresponding logical addresses. The memory controller writes the data of the host command into active blocks of the flash memory. In response to the memory controller changing the active blocks into unsaved data blocks and a number of the unsaved data blocks being greater than or equal to an unsaved data block count threshold, the memory controller segmentally updates mapping relationships of the data in the unsaved data blocks, and writes the updated group-mapping tables into the flash memory.