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When earth is used as the current return path of Ultra High-voltage Direct Current (UHVDC) system, great DC current will flow in the earth, which will bring about great ground potential differences in a large area around the DC grounding electrodes． To investigate UHVDC grounding current, A three-layer earth model combining vertical and horizontal layers is established by analyzing the ground soil structure． Based on Quasi-Newton method, the parameters of soil structure obtained from unequally spaced Wenner four-probe test data are optimally estimated. According to the model, the current and voltage of ground have been calculated around the UHVDC electrode site and used to analyze the leakage situation of grounding current in soil structure of Sichuan Xiluodu Hydropower Station ． The decline of current in super stratum is different because of distance． The choice of UHVDC electrode site is extremely sensitive to terrain and geological condition, and that Xiluodu and Xiangjiaba Hydropower Station are located between Sichuan and Yunnan border area of high altitude, heavily covered ice and large mountainous districts． Finally, some suggestions about the choice of UHVDC electrode site were provided in this paper． Index Terms --Grounding electrodes, Quasi-Newton method, Soil structure parameter, UHVDC