Abstract: A data auto-backup system includes a main server and a terminal device. The main server includes a registered account, and the terminal device includes an application connected to the main server. When the application of the terminal device detects a registered portable storage device connected to the connection port, the application reads a local file stored in the registered portable storage device, and uploads the local file to the registered account in the main server as a cloud backup file. As a result, the terminal device automatically backs up the local file of the registered portable storage device to the main server whenever the registered portable storage device is connected, saving the user from manual backup steps, and preventing valuable data from missing. Furthermore, a software solution to encrypt data in the portable devices is employed without introducing any hardware to the existing portable devices.