The invention discloses a full-automatic tapping machine which comprises a machine head assembly, a clamp, a supporting plate device and a guide rail device. The machine head assembly comprises a motor, a machine base and a screw tap. The motor is fixed to the machine base, and the screw tap is driven by the motor. The clamp comprises a clamp air cylinder, a first connecting rod mechanism, a first linking rod, pressing bars and two first linking rod seats. The two ends of the first linking rod are connected with the two first linking rod seats through bearings. The pressing bars are fixed to the middle portion of the first linking rod. The tail end of the first connecting rod mechanism is fixed to the end of the first linking rod. The clamp air cylinder drives the first connecting rod mechanism so that the pressing bar can clamp or loosen a workpiece to be machined. The supporting plate device comprises a supporting plate, a supporting plate air cylinder, a second connecting rod mechanism, a second linking rod and two second linking rod seats. The supporting plate air cylinder drives the second connecting rod mechanism. The guide rail device comprises two slope guide rail plates and two limiting plates, wherein the two slope guide rail plates are fixed to a worktable, and the two limiting plates are located on the outer sides of the two slope guide rail plates. When a piston rod of the supporting plate air cylinder retracts, a supporting plate is connected with the tail ends of the slope guide rail plates and supports the workpiece to be machined.