A memory management method for managing physical units of a rewritable non-volatile memory module is provided. In the method, the physical units are grouped into at least a data area and a free area. The method includes obtaining empty physical units from the free area to configure a first global random area and obtaining empty physical units from the data area to configure a second global random area. The method further includes using the physical units of the first global random area to write updated data, and using the physical units of the second global random area to write other updated data after the physical units of the first global random area are written full of the updated data. Accordingly, the method can increase the storage space of a global random area, and thereby reduces data merging operations and shortens the time for executing a write command.