A prop for construction. The prop includes an outer tube and an inner tube telescopically fitted in the outer tube. The top end of the outer tube has a threaded section axially formed with a pair of slots. The inner tube is axially formed with several pairs of pinholes. A locating nut is screwed on the threaded section of the outer tube. A controlling member formed with a pair of through holes is fitted on the outer tube. A bottom end of the controlling member is rotatably connected with the locating nut. A top face of the controlling member is formed with recessed / projecting structures. An insertion pin is fitted through the slots of the outer tube and inserted in one pair of pinholes of the inner tube, and is rested on the top face of the controlling member, whereby the prop can support a horizontal moldboard. By means of turning the controlling member, the insertion pin and the inner tube can be quickly lowered to separate the top end of the prop from the moldboard. Alternatively, the insertion pin can be fitted through the through holes of the controlling member, the slots of the outer tube and inserted in one pair of pinholes of the inner tube, whereby the prop is able to bear both axial pressure and tension.