Abstract: To improve the accuracy of the calculation results of influence on AC power system by grounding current when HVDC operates in monopole and grounding loop circuit, this paper presents an calculation method of grounding current of HVDC based on overdetermined linear equations, which makes full use of the measured DC data, and the DC in neural node of transformer caused by HVDC is evaluated by combining of additional equation built by measured data with theoretical calculation model. The overdetermind linear equations model of loop current method and nodal voltage method are derived, and the mathematical model of overdetermind linear equations with least square method is solved to get the DC in the neutral node of transformers. Taking a four network impedance model as example, the method is verified and its results are compared with that obtained by traditional model and method. The results show that this method is simple, practical and high reliability, and its superiority is reflected in such condition as lacking of accurate raw data.