Abstract: Extremum seeking algorithm with sliding mode is presented for design of automatic tuning control system of SSC-LINAC RFQ(Radio Frequency Quadrupole) based on the cavity reflected signal. Combined with the Lyapunov stability theory and the characteristics of SSC-LINAC RFQ radio frequency system, the control gain and reference signal of the sliding mode algorithm are improved. By solving the differential equation, the functions of the cavity reflected signal with respect to time and the position of the tuning rod are obtained. Through software simulation and hardware system design, the tuning process based on extremum seeking algorithm with slidig mode is simulated and tested. The results show that the designed frequency stabilization system can realize the automatic power feeding process of SSC-LINAC RFQ cavity in a short time, and the frequency stability meets the design index requirements, and has passed the long-time stability test. The feasibility of applying the extremum seeking algorithm with sliding mode to the tuning of high frequency resonator is proved.