The invention discloses a focusing device of a prime lens. The focusing device comprises a rotating disk and a guiding disk, which are annularly and coaxially arranged and are rotatably connected witheach other, a focusing buckle is arranged between the guiding disk and the rotating disk, and the focusing buckle is used for performing positioning and matching with a focusing groove of the lens; and the focusing buckle is circumferentially locked with and radially and slidably connected with the guiding disk, the focusing buckle is slidably connected with the rotating disk, and the focusing buckle is driven by the rotation of the rotating disk to move toward the radial direction of the lens to adapt to the focusing grooves of lenses with different diameters. The focusing buckle is driven by the coaxial relative rotation action of the rotating disk and the guiding disk to slide along the radial direction so as to change the relative radial position of the focusing buckle, thereby changing the effective clamping diameter of the focusing buckle, and the focusing grooves of prime lenses with different diameters can be adapted to within a certain adjustment range as a result. The structure that the lens is completely submerged into a lens flange can be fully adapted, and furthermore, because of the stable clamping fit of the focusing buckle and the focusing groove, a sufficient focusing force can be provided without being restricted by a sealing ring.