The DC bias current is affected by many factors, among them, the soil structure is one of the most important factors. In order to analyze the influence of the soil structure on the DC bias current of HVDC transmission system more accurately, a simplified grid model is proposed to analyze the influence of the soil structure on DC bias current. First of all, the calculation method in direct current transmission regulation is applied to verify the method, the DC bias current under several different soil models is analyzed. The results show that the tendency of the result from simplified grid model and the existing research is consistent, and The simplified grid model used in this paper can provide qualitative and quantitative references for the influence of soil structure on DC bias current.