Eight 500MHz cavities are applied in the storage ring of the Shanghai Synchrotron Radiation Factility (SSRF). In order to tune the cavities for different changing parameters and operation modes, the control system is essential for operations. The tuning is achieved by synchronizing the drive signal and the cavity probe signal. The error signal defined as their subtraction is amplified and used to drive a stepping motor, which in turn moves a metallic plunger in or out of the cavity by programmable logic controller (PLC). The tune speed is set to be 1kHz/s.