It is a dispensable step for many electromagnetic compatiblity problems to calculate the mutual impendance between the overhead and the underground conductors in a multi-layered soil. To this end, an efficient approach is proposed in this paper. The expression of the mutual impedance, which contains a complex highly oscillatory semi-infinite integral, is divided into two parts intentionally in the first place, i.e., the definite and the tail integral respectively. The definite integral is calculated using the proposed moment functions after fitting the integrand into a serial of spline functions, and the tail integral is replaced by an exponential integral with an asymptotic integrand. The advantage of the proposed approach is that the calculation of oscillatory integral is completely avoided due to the fact that the moment function is evaluated analytically. Numerical examples justify the proposed approach in many cases with different combination of parameters. The proposed approach can be used in the electromagnetic compatibility analysis between overhead and underground conductors.