The Stewart machining robot adopts a closed ring structure; the shaft was connected with a number of spatial links. In the coordinate system fixed on the cutter, the coordinates of the hinges joining control links and shaft can be determined. And the posture of the tool can be determined as well. Corresponding to every posture of machining, the position of shaft can made the lengths of the two Control axes equal. So that if one of the controlling ends of NC system controls two axes simultaneously, a 5 coordinate system can realize the function of the 6- coordinate system.