In frequency spectrum IP method, the application of residual electroma gnetic effect to the study of the distribution of underground electricity has aroused much interest. Based on an analysis of numerical calculations, this paper points out that, when the high frequency f_G is properly chosen, the phaseφ_s(f_G) of apparent complex resistivity is observed by dipole device, and it is normalized by the phase theoretical valueφ_s⁰(f_G) of the apparent complex resistivity of uniform ground under the corresponding condition, then the ratio φ_s(f_G)/φ_s⁰(f_G) obtained is the ideal parameter representing residual electroma gnetic effect. Combining numerical calculations with field data, the paper also makes an analysis of the anomalous character of this parameter and its effect in the prospecting for oil and gas.