The theory of micro dimensional behavior on rolling mill is used for a typical 450 mm hot plate four-high mill. A statically determinate device is adopted on the back-up roll system to adapt to the micro-dimensional deflection. We replace the traditional single base with the force divided base for the rolling load. To make the rolls parallel, a worm-screw device is set in the chock, with a clearance between the chock and the stand window to allow convenient roll replacement and retain a thermal expansion space. In the new mill, the center line of the roll body is constant when the rolling is stopped or running. It can be predicted that the mill will be strengthened two times and the roller bearing life will be prolonged ten times.