According to the chatter marks on steel strip and backup roll of 1220HC rolling mill，continuous tracking measurement is carried out. On the basis of researching the mill inherent characteristics，vibration signals in different periods of severing backup rolls are compared，and it is found that，in early stage，the main vibration frequency is 568 Hz which is the 7th natural frequency of the rolling mill，and the relative movement between the middle roll and the backup roll induces chatter marks on the surface of backup rolls in steady rolling stage. In middle and later stage，the backup roll chatter marks react on rolling mill，and the forced vibration is formed. As a result，the vibration model is a coexistence of forced vibration and resonance. Based on the above studies and the relationship of mark spacing and vibration source，the cause of the 7th natural frequency is found and the effective countermeasures are put forward.