The design theory of a statically determinate rolling mill, including the redesign of the statically indeterminate characteristic and hyperstatic characteristic, is proposed in this paper. The statically indeterminate characteristic leading to the nonparallel running state of rolls is revealed based on analysis of three rolling mills, including a 2200 mm aluminum foil four-high mill, a 1580 mm PC four-high mill, and a 650 mm bar two-high rolling mill, etc. The hyperstatic characteristic leading to uneven load performance of four-row roller bearing is caused by redundant constraints in the roll system based on analysis of 17 two- and four-high rolling mills. In contemporary four- or six-high plate rolling mills, regular dynamic cross between the rolls is very common during the whole rolling process. Dynamic cross results in excessive axial forces causing frequent burning losses of the end-thrust bearings and leads to serious deviations in the rolling load on both ends of the backup roll. Hence, rolling mills designed for heavy loads and high speeds have yet to operate at their design load and speed in a safe and stable way. According to the analysis of the degrees of freedom of a spatial mechanism, the cause of dynamic cross lies in the statically indeterminate characteristic of the roll system caused by the small clearance in the system. In this paper, the theoretical analysis and experiments invalidate the accepted method of the offsetting of the mill. Furthermore, it is shown that offsetting causes an asymmetrical cross between the rolls. Therefore, it is proposed that offsetting should be eliminated, and additional jack devices should be introduced to maintain the rolls parallel. In addition, bending deflections of the roll induce hyperstatic characteristics of the roll system due to the existence of redundant constraints in the system. The hyperstatic characteristic of the roll system results in uneven load performance of the radial roller bearings and shortens bearing life. In the newly proposed statically determinate four- or six-high plate rolling mill, which can be operated in a safe and stable way under heavy load and high speed working conditions, this problem can also be solved.