A locking and unlocking nut structure contains a body including a through hole, a diameter of the through hole being larger than that of a screw rod (tube), the body also including a semi-circular recess with a starting segment and a stopping segment, the body further including a slot and an diameter-decreased screw section; a slidable block retained in the semi-circular recess and moving between the starting segment and the stopping segment, the slidable block including first threads, while the slidable block is fixed on the starting segment, a distance from the first threads of the slidable block to an inner wall of the through hole is larger than an outer diameter of the screw rod (tube); while the slidable block is fixed on the stopping segment, a distance from the first threads to the diameter-decreased screw section is equal to the outer diameter of the screw rod (tube).