Abstract: Based on the special setup mode of the rotary expanding rolls, a set of basic formulas for the rotary expanding is deduced, and the formation and changing characteristics of the feed angle is analyzed. The results demonstrate that: although the roll setting in rotary expanding process doesn’ t directly present the formation of the feed angle, the toe angle and the vertical movement of the rolls jointly lead to producing a feed angle, which makes the rotary expanding take on typical cross rolling characteristics; that the bigger the toe angle, the bigger the feed angle; the bigger the vertical movement distance of the rolls, the bigger the feed angle; the smaller the distance between rolling center line and the center of the basic circle, the bigger the feed angle; and that from the entrance to the exit of the deformation zone, the feed angle of each pass point on the roll surface changes widely from 7.5° to 3.0°.