Abstract: A quick-action cylinder with safety device to prevent blocking is described, wherein the quick-action cylinder provides a mechanical lock for a center draw-in nipple (2) in a center bore (23) in a housing of a quick-action cylinder, wherein in the locked position one or more locking members (3) are urged into contact with an exterior periphery of the draw-in nipple by a force of a spring assembly (12) which is arranged in the housing in one or several spring compartments (13), and wherein the unlocked position of the draw-in nipple (2) is attained by applying pressure to a locking piston (7) which operates to oppose the force of the spring assembly. The safety device prevents self-blocking of the locking device of the draw-in nipple (2) in the center bore (23) when the pressure medium enters the spring compartment (13) of the spring assembly (12) from the cylinder space (11) of the locking piston.