This article proposes a novel fault diagnosis method to detect the fault of high-voltage capacitor (HVC) for dc filter (DCF) in high-voltage direct current (HVDC) systems. The proposed diagnosis principle is developed through identifying the change of HVC capacitance, which is calculated in real time using the measured voltage and current of DCF. Compared with the traditional methods, the proposed approach is superior in detecting successive faults and symmetrical faults since any HVC fault will change the capacitance parameter of HVC essentially. Simulation and experimental results verify the performance and superiority of the presented method.