Up to present, how to reduce more effectively the rolling straightness error of the missile cylindrical hull plate (called as the missile plate for short) is a difficult problem attracting the gaze of people. Systematical calculation, analysis and research on the elasticity-plasticity mechanical relationships between the missile plate and the three rollers in the symmetrical type of three roller bender for rolling missile plates (called as the bender for short) were made by the author. In this article, on the basis of concerned research results obtained previously, further exploration and deeper research on the source causing the straightness error of the missile plates under actual condition are put forward, and a group of equations for determination of the radii of the improved type rollers are derived. It is proved that replacing the original rollers in the bender with the improved type ones is able to reduce effectively and notably the rolling straightness error of the missile plate.