The online identification of the filter detuning elements has an important practical value for improving the reliability of the HVDC system. This paper presents a novel method for online identification of the detuning elements of the DC-side filter of the HVDC system. The method only needs to measure and extract the harmonic components from the current that through the single-tuned filter. Weather the filter has detuned or not can be known by the analyzing the change rate of the harmonic current under resonant frequency. In the situations that the filter has detuned, the detuned elements can be identified by calculating the ratio of the change rates of several special harmonic current. This method can be realized based on a simple modification of the traditional DC-filter, and thus the function of the filter is expanded. This paper presents the principles of the proposed method and the simulation results of the application of this method on different filters verify the validity and effectiveness of the proposed method.