A garbage collection method for a data storage device includes steps of: entering a background mode from a foreground mode; selecting a plurality of source data blocks from a plurality of in-use data blocks; dividing a mapping table into a plurality of sub-mapping tables and selecting one of the sub-mapping tables as a target sub-mapping table, wherein the target sub-mapping table is used to manage one of the source data blocks; selecting a destination data block from a plurality of spare data blocks; and sequentially updating a correspondence relationship of data stored in the target sub-mapping table from the source data blocks to the destination data block, wherein the updating comprises copying the data stored in the source data blocks to the destination data block.