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This paper proposes a new method of finite-difference numerical modeling using interpolation to achieve boundary condition. Surface function can fit the true shape of geographywell through 3 times spline interpolation. The interpolation method of surface boundary condition is to find a point in the normal line of surface which is located in its same equipotential line, and sets up discrete equation through the interpolation by around points. Irregular quadrilateral distance weighted interpolation is used to make interpolation models unified. Comparative analysis of numerical solution and analytical solution in level surface shows that the calculation accuracy of thismethod is qualified, and the result of examp le calculation for valley model shows resistivity and topography as mirror symmetric.