Current injection method for transformers' DC bias under HVDC transmission ground return mode is presented. A DC network model of a power grid is set up to analyze the mechanism behind the proposed method. By comparison between simulation and measurement， validity of the method is checked and the impact of soil resistivity and compensation distance on compensation degree is analyzed. Simulation and application of the method in Yihe Substation shows that: the method actually functions as potential difference compensation to the bus of substation; higher soil resistivity demands longer compensation distance and higher injection current， but it leads to lower compensation degree and the compensation degree usually bellows 0.6 . At the end， the example of Luodong Substation is listed to verify that current injection method is not applicable to substations under complex operation mode， then another solution is brought up.