In a 1220mm temper mill, the cross bow was serious, which seriously affected the unit’s output and efficiency. In order to solve this problem, the mechanism of cross bow is studied in rolling of single-roll transmission temper mill. First, established the FEM of the single-roll drive temper mill rolling, analyzed the distribution factors of vertical extension in thickness. Second, used the method of symplectic elasticity to establish the analytical calculation model to study the relationship between the cross bow with the uneven vertical extension, explained the mechanism of cross bow. Then, used ANSYS to establish finite element model which further analyzed the cross bow under different distribution of extension in width direction. On the basis of analyzed the mechanism, proposed measures to reduce the value of cross bow. These measures have been applied in production; the effect of application was obvious.