An expandable computer system and a fastening device are disclosed in this invention. The expandable computer system includes a computer, an expansion device and a rigid component. The expansion device includes a first holding portion disposed at a first side of the expansion device, and a first interface socket disposed at a second side of the expansion device. The computer includes a second holding portion and a second interface socket. The rigid component has two transmission plugs and a transmission circuit. The two transmission plugs are coupled with two terminals of the transmission circuit. When the first holding portion is connected with the second holding portion, the two transmission plugs are detachably connected with the first interface socket and the second interface socket to fasten the expansion device to the computer.