A method for memory space management is disclosed. It uses a resident program loaded into an operation system or the controller of a storage device to monitor the storage space and the resource allocation of the file system of the storage device. The status of the logical address with an erased and invalid data mapped with a physical block is checked via a L2P mapping table. By using a data erase instruction, the controller modifies the L2P mapping table to cancel the link relation between the physical block and the logical address and erase the physical block to release the memory space. Finally, the check location is stored for a next check. The method for memory space management improves the access speed and the usage life of the storage device.