As an important facility of high-voltage direct current (HVDC), site selection and design of DC grounding electrodes have always been the focus of research on HVDC. The aim of the paper is to explore the method of calculating the Green’s function of wide-area layered earth in complex geological structure. The location of DC grounding electrode is completed, and the risk of DC bias in AC power grid is evaluated by simulation. Firstly, the characteristics of complex wide-area earth in the northeast area of Jiangxi province are clarified. Secondly, the Green''s function of surface potential on the wide-area earth is deduced, and the results of earth surface potential calculated by the commercial calculation software CDEGS and the improved complex image are compared and analyzed. It is found that the improved complex image is not limited by the number of horizontal layers of soil, so it has a broader application space. Finally, the DC distribution of large complex power grid is established to analyze the DC bias risk of substations with different grounding electrodes, then choose the site of grounding electrode with less risk as the final plan.