An air guide device is provided on a side panel of a computer housing, and includes an air guide member and a positioning mechanism. The air guide member includes a first coupling portion and a second coupling portion disposed opposite to the first coupling portion for coupling with the first coupling portion. The positioning mechanism includes at least one positioning member for coupling a bottom portion of the air guide member to the side panel. The air guide member is convertible between a state of use where the air guide member is in the form of a hollow cylinder, and the first and second coupling portions are coupled, and an unfurled state where the air guide member is in the form of a flat sheet, and the first and second coupling members are separated.