UHVDC transmission system is characterized by transferring large amount of electrical power over longdistance , which plays an important role in the west-to-east power transmission project. Since the underground current can reach several kA under UHVDC monopole operation, the safe and stable operations of grounding electrode are highly relative with nearby soil resistivity. This paper introduces porous media model and analyzes the influence of rising temperature around grounding electrode on soil resistivity. Based on researching soil conduction mechanism under DC, calculation formula for soil resistivity and the rule of DC density on soil resistivity are presented. Furthermore, the relationship between DC density and soil resistivity is obtained, which was demonstrated by currenr characteristics experiment of soil resistivity.