A height-adjustable apparatus for supporting a flat monitor is disclosed. The height-adjustable apparatus includes a first support unit, an elastic element, and a second support unit. The first support unit has a first engaging device. One end of the elastic element is affixed on the first support unit. The second support unit, which is movably sleeved into the first support unit, has a second engaging device, a first end, and a second end. The first end touches the elastic element, and the second end connects the flat monitor. When the first engaging device and the second engaging device engage with each other, the height-adjustable apparatus has a first length. When the elastic element is extended with an external force, the second support unit slides to a position such that the apparatus has a second length. The first length is longer than the second length.