This paper discussed the different simply approximate calculation methods in applying the model theory to the excitation function approach in calculation of the electrical field of radio interference. If assuming that the electromagnetic field generated by radio interference propagates in the homogeneous medium, the parameter matrix of impedance and admittance will have simply forms and the components of propagate coefficient satisfy special relationship. When the earth effect is taken into account, the process become more complex, but be more accurate. Based on different assumption, the paper compares the result of the radio interference calculated in the excitation function approach of common lines, the result shows that the bias are acceptable.