  Tuning system plays a very important role when superconducting cavity works.
It cooperates with other control loops, to adjust the frequency of cavity with
high precision, reduce the reflection power, guarantee the stability of beam
and ensure the safety of superconducting cavity. This paper focuses mainly on
the tuning system working principle, the working state and problems that BEPC
(Beijing Electron Positron Collider) II has encountered during operation.
