A locking and unlocking nut structure comprising: a body including a through hole passing therethrough axially, a diameter of the through hole being larger than that of a screw rod (tube), the body also including a semi-circular recess defined on a top surface thereof, and the semi-circular recess having a starting segment defined on one end thereof and a stopping segment defined on another end thereof, the body further including a diameter-decreased screw section defined on an inner wall of the through hole thereof opposite to on an inner rim of the stopping segment of the semi-circular recess; a slidable block including a C-shaped cutout fitted on the semi-circular recess, and the C-shaped cutout including an engaging hook extending outward from a lower end thereof so that the engaging hook retains a lower end of the body to move the slidable block between the starting segment and the stopping segment of the semi-circular recess, the slidable block including first threads defined on one of two outer surfaces thereof, while the slidable block is fixed on the starting segment of the semi-circular recess, 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 of the semi-circular recess, a distance from the first threads to the diameter-decreased screw section of the through hole is equal to the outer diameter of the screw rod (tube); the screw rod (tube) including second threads to match with the diameter-decreased screw section and the first threads of the slidable block. a body including a through hole passing therethrough axially, a diameter of the through hole being larger than that of a screw rod (tube), the body also including a semi-circular recess defined on a top surface thereof, and the semi-circular recess having a starting segment defined on one end thereof and a stopping segment defined on another end thereof, the body further including a diameter-decreased screw section defined on an inner wall of the through hole thereof opposite to on an inner rim of the stopping segment of the semi-circular recess; a slidable block including a C-shaped cutout fitted on the semi-circular recess, and the C-shaped cutout including an engaging hook extending outward from a lower end thereof so that the engaging hook retains a lower end of the body to move the slidable block between the starting segment and the stopping segment of the semi-circular recess, the slidable block including first threads defined on one of two outer surfaces thereof, while the slidable block is fixed on the starting segment of the semi-circular recess, 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 of the semi-circular recess, a distance from the first threads to the diameter-decreased screw section of the through hole is equal to the outer diameter of the screw rod (tube); the screw rod (tube) including second threads to match with the diameter-decreased screw section and the first threads of the slidable block.