The invention introduces a method for GC (Garbage Collection) in a flash memory, performed by a processing unit, including at least the following steps: reading n×m pages of good data from storage sub-units, wherein n indicates the quantity of storage sub-units sharing one channel and m indicates the quantity of pages for programming data into one storage sub-unit; and repeatedly performing a loop for directing each of the storage sub-units to program m pages of good data until all of the storage sub-units are operated in busy states.