In order to accurately solve the current field of HVDC under unipolar operation or malfunction

a morepractical model should be established. In this paper

the specific dimensions of the grounding electrode are taken in-to account

and the size of the earth model is increased to thousands of kilometers

so that the model is closer to reali-ty

and more accurate results can be obtained. Considering the enormous size difference between the electrode andthe earth area

the model belongs to the multi-scale model. For the multi-scale model

the overlapping domain de-composition method is adopted to decompose the small domain including the ground electrode from the whole model

and each model is solved independently. The potential on the virtual boundary is taken as iterative variables

and thewhole problem is finally transformed into an asymmetric real number linear equations. For the iteration of domain de-composition

based on the Arnoldi’s orthogonalization and the mathematical principle of generalized minimal residu-al method（GMRES）

the GMRES iterative algorithm for not specifically solving the coefficient matrix of linear equa-tions is derived. It is proved that the GMRES iterative algorithm is more efficient and feasible than the direct and suc-cessive over-relaxation iterations. Finally a more accurate calculation of HVDC grounding current field is achieved

by comparing with the existing literature that the specific size of electrode has certain effect on the calculation re-sults. Algorithm in this paper provides a new method to calculate multi-scale constant current field.