A combined screw tap comprises a screw tap handle, a screw tap joint, three screw tap joint dovetail grooves, screw tap blades, screw tap chip grooves and screw tap chip groove blocking heads, wherein the screw tap is divided into a main body part and a blade part, the main body part is further divided into the screw tap handle and the screw tap joint, and the screw tap handle and the screw tap joint are integrated; and the three dovetail grooves in the axial direction are evenly distributed in the screw tap joint in the radial direction, the screw tap blades are connected into the dovetail grooves in an inserted mode, the chip grooves are formed between the dovetail grooves, and the blocking heads are arranged on heads of the chip grooves; and the tails of the chip grooves are in oblique transition with the screw tap handle.