The sensitivity analysis reveals the influence of triple-tuned DC filter component parameters on its resonant frequency. Based on that finding， a detuning detection method for triple-tuned DC filter is proposed according to the rough-set theory. In this method， a decision-making table is established， the relative change of harmonic current flowing through DC filter caused by various parameters is condition attribute， and the component type is decision attribute. Detection rules of fault component from the established decision-making table are derived. Finally， the rules of evaluation on the fault location， like the certainty factor， coverage and confidence are discussed. A large number of simulation on the actual DC system shows that the fault locating method is simple and effective.