A garbage collection method for a flash memory is provided. The flash memory includes a spare block pool and a data block pool, wherein the spare block pool includes spare blocks and the data block pool includes data blocks. The method includes the steps of: receiving target data from a host and writing the target data to a current data block of the data blocks; sorting an erase count of each data block when performing a wear-leveling process to write the target data; sorting a valid page number of each first block when it is determined that at least two first blocks in the data blocks have the smallest erase count; and selecting a second block having a smallest valid page number from the first blocks and writing valid pages of the second block to one of the spare blocks to perform a data cleaning process.