The present invention is to provide a system for backing up cache memory in a double backup server structure, which comprises a first backup server having a first SAS controller; a second backup server having a second SAS controller; and a SAS channel for connecting said first and second SAS controllers; wherein said first SAS controller and said second SAS controller each having a sending end initiator defining a cache mirror command. When said first backup server received a write command and write data through a Fiber Channel or iSCSI channel into a first cache memory, said first SAS controller also establishes a first Command Descriptor Block (CDB) with an operation code of a cache mirror command as well as a first data header for the cache data. Said first CDB and first data header are subsequently transmitted through said SAS channel to said second SAS controller and said second backup server writes said data into a second cache memory of its own. Accordingly, with the higher bandwidth facilitated by the SAS channel, considerable amount of cache data can be processes in high speed to improve the storage efficiency in a double backup server structure.