This paper proposes a novel method for the HVDC system， which can detect the system fault and， more importantly， diagnose the HVDC fault function. First of all， a fault detecting filter is constructed to detect the system faults. Based on the LMI approach， the design of the fault detecting filter problem can be transformed into a problem for system robust stability analysis. In order to classify the detected HVDC faults， a novel iterative-learning-algorithm-based fault observer is developed to diagnose the HVDC system faults. The simulation results indicate the effectiveness and efficiency of the approach proposed.