Mechanism of roll forming for the extra-thick plate is more complicated, which is greatly influenced by the thickness characteristics of extra-thick plate and affects the machining accuracy of the extra-thick plate. In order to improve the forming quality of the three-roll coiling machine for the extra-thick plate, a stress-strain model with a linear distribution of the thickness is established. Based on the analysis of roll deformation of extra-thick plate, the analytical model of the three-roll rolling process is established. The stress field distribution in the bending deformation zone was analyzed with the use of ABAQUS software. The influence of the pressure on the upper roller, the friction between the roll and the sheet, and the processing speed of the sheet on the forming of the plate were analyzed. The results show that compared with the medium-thick plate, extra-thick plate appeared more obvious stress concentration in the contact area between the roll and the plate and the maximum equivalent stress increased by 15.3%. When the upper roller moves down the same distance, the roll forming curve of the extra-thick plate is basically consistent considering the thickness characteristics or not, and the maximum deviation of the forming radius is 5.7%. The forming radius is constant when the roll processing speed is between 0.15 and 0.45 rad/s, and the forming radius of the extra-thick plate is larger than that of the 30 mm medium-thick plate by 2.3%. The forming radius of the plate increases with the increase of friction coefficient between the rollers and plate. Compared to the 30-mm medium-thick plate, the forming radius of the extra-thick plate increases by 20%.