A method of automatically adjusting a size of a copy-on-write (COW) disk space of a snapshot device is provided. A first disk space of a snapshot device is initialized, and a COW operation is performed on a chunk of the first disk space. Next, it is determined whether a chunk sequence number of a write request is in the first disk space. Then, if the chuck sequence number of the write request is in the first disk space, the first disk space is maintained. Then, if the chuck sequence number of the write request is not in the first disk space, a second disk space is initialized, and the number of chunks of the second disk space is the same as the number of maximum successive chunks of the first disk space, and then, the COW operation is performed on the chunk of the second disk space.