In a backup method, a source storage device sends data to a backup storage device. The source storage device contains a processor and a cache. The processor receives a write data request which includes target data. And then, the processor reads a first period ID recorded in a period ID table, wherein the first period ID is corresponding to a first period. Next, the processor modifies the write data request by attaching the first period ID to the target data and writes the modified write data request into the cache. After a backup task corresponding to the first period is triggered, the processor identifies a second period ID. The second period ID corresponds to a previous period up to which data received by the source storage device have been duplicated to the backup storage device. And then, the processor obtains data received after the period corresponding to the second period ID and up to the first period and sends the obtained data to the backup storage device.