Abstract: P to now, how to reduce more effectively the large rocket hull straightness error has been an interesting problem for technicians of manufacturing rocket hulls. On the basis of systematical analysis, calculation and investigation on the distribution law of the center roller feeding value over the axis of itself under the most disadvantageous practical operation conditions, an inequality for calculating the straightness error of the large rocket bull formed by the bender with three rollers is derived by the author. Relationships between the straightness error of the large rocket hull and geometrical parameters of three rollers in the bender are shown by the inequality. According to the results of studying and comparing the relationships, the author indicates that improving the bender with three rollers is the important way for reducing the straightness error of a large rocket hull. It has been proved by a series of researches that improving the bender with three rollers by use of the method recommended in this paper can make the large rocket hull straightness error reduced considerably.