Beam lifetime is dominated by Touschek scat-tering at the Shanghai Synchrotron Radiation Facility (SSRF). Touschek loss rate is affected by probability for scattering beyond the RF acceptance and the volume charge density of the bench. In the phase II upgrade of the SSRF, a third harmonic superconducting cavity will be used to enhance the Touschek lifetime by lengthening the bunches. The Touschek lifetime improvement factor is affected by the voltage of a harmonic cavity. To stabilize the cavity voltage, a tuning control system was designed to control it. The design of the tuning control system was based on the SSRF third-generation low-level RF control system. Some hardware and specialized algorithms were redesigned to fit the harmonic cavity control. The design of the tuning control system is complete, and the control system has been tested. The test result shows that the fluctuation of amplitude is± 0.34% within 1.5 h, which satisfies the stability requirement.