With the inaccurate prediction of surface voltage, the assessment of the DC-biased current may deviate greatly from the actual value. In view of the problem, the surface potential expression of multi-layer soil is obtained by combining the soil boundary conditions with the mathematical analytic method, based on Laplace equation and Poisson equation under the relationship of field source. Then the reasonable soil model is determined by analyzing the surface voltage distribution curves of three soil models. On this basis, the transformer neutral point DC evaluation model is established by the method of direct field-circuit coupling. Finally, the surface voltage distribution prediction and the DC-biased current calculation of transformer neutral point for a regional power grid in Shanghai are realized based on .NET platform. The validity and accuracy of this model are proved by comparing the calculation results with the simulation results of CDEGS software.