It is an important technique for high-speed milling die free-form surface with ball-end cutter.On the basis of geometric position relation between the machining point of the ball-end cutter and the machining curve,the motion law of the machining point was discussed in machining with ball-end cutter.And the linear velocity and the angular velocity equations of the milling point were obtained in machining die free-form surface with the ballend cutter,which provided necessary theoretical basis for the study of uniform wear of the ball-end cutter,and provided reference for the ultra precision machining of free-form surface.