Conical screw rotor is the key part of conical double-screw extruder, with complex geometrical features, such as variable outside and inside diameter, unequal groove depth, varied head and lead. The manufacturing process is complicated and difficult. The precision and efficiency of conventional machining method is lower, the mating character of machined screw rotor is bad and the exchangeability is hard. On the basis of studying the geometric character and contacting process of double conical screw, this paper presents a new NC manufacturing method to envelope milling screw rotor with several axes moving combined. The automatic programming system for screw rotor NC machining is developed. This research increases the machining level of conical double screw. It has conspicuous engineering meaning.