The background and relevance of HVDC technology development for Russian electric power system is caused by its operating features and functioning conditions of the Russian Unified Power System (UPS). Using HVDC systems can solve a number of problems, existing nowadays in Russian UPS, such as power supplement of oil platforms and territories accessed through the water barriers, the connection of renewable energy sources to the UPS and to local power grids and others known problems. The relevance of the discussed issue is caused by the need of improving the accuracy of the fault location in the HVDC transmission lines and by the absence of that line fault location (LFL) devices in Russian energy development practice. The main aim of the study is to improve the accuracy of the spectrum method algorithm for LFL issue for the HVDC lines; to develop the formula of the unknown fault distance determination relying on the transient frequency, HVDC line parameters and the length of HVDC line faulted segment with consideration of a unfaulted segment in the equivalent circuit of the HVDC system. The methods used in the study. The authors have used the analytical research methods for determining the formula, which allows solving the problem of LFL in DC line using spectrum method. While investigating the accuracy of the proposed LFL spectrum method the mathematical modeling techniques were used. The results. The authors proposed the LFL spectrum method solving the issue of fault distance determination in the HVDC line containing cable and overhead segments. This method considers the HVDC line containing also smoothing reactors and high harmonic filters. The authors proposed as well the equivalent circuit of the HVDC line unfaulted segment - the equivalent line lumped model. The formula to determine LFL distance, in dependence of the main frequency in the transient voltage spectrum, line parameters and equivalent circuit parameters, was obtained. Inaccuracy of the proposed method, injected by the undetermined value of transient resistance, was estimated.