In this paper,a large four-roller plate bending machine is used to roll a batch of shells with same material different sizes and same size different materials for two groups of research objects. Data of the thick-shell rolling length variation in the rolling process are collected and analyzed. It is concluded that the plasticity of materials and the shell sizes are the main reasons for the rolling length variation. For the different materials,better plasticity means easier roll long,more deviation and greater impact on the shell forming accuracy. For the same material,larger thickness and smaller diameter mean greater pressure on the shell in the rolling process,more plastic deformation and greater impact on the shell forming accuracy. Through this experimental study,we summarize the experience for optimizing the rolling process,controlling the change of rolling length,and improving the thick-shell forming accuracy.