An adjustable joint for a flat object is disclosed. The adjustable joint includes an extending portion connected to the flat object, a fixed portion, and a resilient portion connected to the extending portion and the fixed portion. The resilient portion includes a joint member connecting the extending portion and the fixed portion, a latching member received in the joint member and latching the extending portion, an operating lever rotatably connected to the joint member and configured for adjusting the tightness of the extending portion and, a connecting member connecting the latching member and the operating lever, wherein when the operating lever is rotated, the connecting member drives the latching member to grasp or release the extending portion.