Under high frequency operating conditions, the resonance of the capacitive power transfer (CPT) system is highly sensitive to parameters, so online tuning is required to maintain the resonance of the system. However, it is difficult to directly obtain the induced voltage on the secondary side of CPT system, so the self-tuning of secondary side cannot be simply realized. This paper proposes a self-tuning method for the secondary side of CPT system using a third pair of decoupled plates and variable inductor. A three-port capacitive coupler with decoupled secondary side is established. A method for judging the resonance state of the secondary side of the CPT system is proposed based on the phase difference detection, and a controller for secondary resonance adjustment based on a variable inductor is carried out. Finally, an experimental prototype was constructed. By adjusting the inductance value of the variable inductor, the equivalent U-I phase difference of the secondary side is reduced from -22.4° to 1.9°, the system output power is maintained at 50 W, and efficiency is maintained around 80%.