This paper investigates the axial dynamic characteristic, under crossing of roll axes of 4-h strip mills, and establishes a reasonable dynamic model of axial vibration of roll system. Based on the predisplacement theory of tribology, the definition of axial rigidity between work roll and backup roll, and between work roll and rolled piece, is given.Axial damping characteristic of roll system is analyzed, and the scale of axial damping factor is established. The analysis of dynamic load amplification factor provides a reasonable basis for discussing the effect of axial force on roll bearings.