A hard disk mounting device includes a receiving portion, a first adapter card, and a second adapter card. The receiving portion includes a bottom plate with a first end and a opposite second end. The first adapter card is attached to the first end of the bottom plate. The first adapter card includes a first connector located at a side of the first adapter card facing away from the second end of the bottom plate. The second adapter card is located at the center of the bottom plate, the second adapter card includes at least one connector e for connecting to a hard disk. The bottom plate includes at least one part for mounting hard disk, each part includes a first fixing mechanism for fixing a hard disk of a first size and a second mechanism for fixing a hard disk of a second size.