A locking device including a hoop, the hoop curling up to form an axial mounting hole in the middle. An upper convex block, a lower convex block, a middle aperture, a left aperture, a right aperture, an upper convex ear, a lower convex ear, an upper guiding inclined plane, and a lower guiding inclined plane are disposed in the device. The upper convex block and the lower convex block move axially to narrow the middle aperture, the upper guiding inclined plane and the lower guiding inclined plane contact with each other to make the upper convex block and the lower convex block to move circumferentially to narrow both the left aperture and the right aperture and enable the hoop to tightly hold objects inside the axial mounting hole.