The fourroll bending machine is often affected by many factors in the actual production, so that there is a big deviation between bending results and theoretical study resulting in a significant reduction in the forming accuracy. Among them, the error phenomenon that has the greatest influence on the forming quality is dislocation. According to the objective law of dislocation phenomenon in production, the types of dislocations were divided into three types: crossshaped type, twisted type and skewed type, and the influencing factors of dislocations phenomena were summarized as the centering deviation, the deviation of the forming path and the onesided starting deviation. Then, the influence law of each factor on the dislocation phenomenon was found by simulation analysis, and the simulation analyses were verified by experiments for the typical dislocation factors. It is confirmed that the single pass forming error is up to 27 mm, and the multipass forming error is only 5 mm. Furthermore, a kind of bending machine with centering device was designed to reduce or even eliminate the impact of various factors on the dislocation phenomenon, and the purpose of improving the forming accuracy was achieved.