The invention provides an automatic screw locking mechanism applied to a moulded case circuit breaker base. The mechanism comprises a first air cylinder, a second air cylinder, a locking mechanism, ascrew chuck and a supporting frame; the supporting frame comprises a sliding rod, an upper supporting plate and a lower supporting plate, an upper dual-ball bearing, a lower dual-ball bearing, a top plate and a transverse frame are arranged on the sliding rod, the first air cylinder is arranged on the top plate, the upper dual-ball bearing is connected with the upper supporting plate, a first aircylinder piston rod is connected with the upper supporting plate, the second air cylinder is arranged on the transverse frame, the lower dual-ball bearing is connected with the lower supporting plate,a second air cylinder piston rod is connected with the lower supporting plate, the screw chuck comprises a shell and a screw feeding port, a first through hole is formed in the upper end of the shell, a second through hole is formed in the lower end, two screw limiting parts, two screw correcting parts and two fourth pin shafts are arranged in the shell. A two-stage universal joint transmission mechanism and the screw chuck enabling screws to be automatically aligned are adopted, the screws can be automatically aligned with locking holes, and the aims of automatically increasing screwdriver rotating torsion and automatically locking the screws are achieved.