A block grouping method includes the following steps. Firstly, a link list is established. In the link list, plural blocks are classified into plural groups according to valid data counts of respective blocks. If a host refreshes a stored data of a flash memory of the solid state drive or adds a new data into the flash memory, the valid data count of the block corresponding to the refreshed data or the new data is changed, and the link list is updated according to the changed valid data count of the block. After the garbage collection is started by the solid state drive, the block to be subject to the garbage collection is selected according to the link list, and the garbage collection is performed.