A method of managing a garbage collection (GC) operation on a flash memory includes: setting a GC starting threshold, wherein the GC starting threshold indicates a predetermined spare block number that is higher than a target spare block number of spare blocks maintained by a flash translation layer (FTL) of the flash memory; determining whether to start the GC operation according to a current number of spare blocks in the flash memory and the GC starting threshold; and performing the GC operation on a source block in the flash memory when the current number of spare blocks is lower than or equal to the GC starting threshold.