A screw rod self-locking device includes a one-way bearing and a mounting seat, and an inner hole of the one-way bearing is sleeved at an end of a screw rod. A mounting hole for receiving the one-way bearing is disposed on the mounting seat, and the one-way bearing is rotatably fitted into the mounting hole along a circumferential direction. An adjusting mechanism for adjusting a circumferential locking force of the mounting hole for the one-way bearing is disposed on the mounting seat so as to adjust a size of a friction force between an outer wall of the one-way bearing and an inner wall of the mounting hole. A height-adjustable leg and a height-adjustable desk applying the same are further provided. In this way, a size of a self-locking friction force to be overcome during backward rotation of a screw rod can be self-defined, thereby guaranteeing consistency of product performances.