During the long service life of an LLC converter, resonant capacitor aging can change the operating point of the switching frequency, leading to significant efficiency degradation. A capacitance estimation method based on discharge large signals is applied for online monitoring of resonant capacitance, which is compatible with the burst mode and pulse frequency modulation of the converter and has no effect on the operation of the converter. And then, a switch-controlled capacitor (SCC) is applied to resonant capacitance compensation, which is triggered when the resonant capacitance degrades by a certain percentage. Experiments verify the accuracy of the capacitor degradation monitoring of this scheme to be more than 98%, as well as the improvement of the efficiency of the converter by the compensation scheme.