An embodiment of the invention provides a backup method for a portable device to back up a first data to a backup server. The backup method includes steps of determining whether the backup server can be accessed; when the backup server can be accessed, establishing a first data transmission path that the first data would be backed up to the backup server via a third party, a second data transmission path that the first data would be backed up to the backup server via a router, and a third data transmission path that the first data would directly backed up to the backup server; selecting one data transmission path among the first, second and third data transmission paths; and backing up the first data via the selected data transmission path.