A connecting device for interconnecting a vertical tube and a plurality of display cabinets for carrying display modules includes first and second connecting frames and a fastening unit. The first connecting frame is for mounting the display cabinets thereon, abuts against a front outer surface of the vertical tube facing the display cabinets. The second connecting frame abuts against a rear outer surface of the vertical tube facing away from the first connecting frame. The fastening unit includes first fastening sets fastening the first and second connecting frames together so as to clamp the vertical tube therebetween. The second fastening sets fasten securely the display cabinets to the first connecting frame.