The present paper deals with the definition and algorithm of impedance real part equivalent resistivity in horizontal electric double source frequency electromagnetic sounding.The equivalent resistivity calculated with this method is the monodrome fundion of electromagnetic respome, which greatly improves the relational curve showing variation of apparent resistivity with frequency, and can intuitively reflect the objective variation of the geoelectric section with depth. The numerical value is relatively close to the real resistivity of strata,with the approximating spsed being very fast, and the effects of false extremes are obviously reduced. Numerical calculation and practical application prove the method to be flexible and feasible, and can satisfactorily eliminate the distortion effect of the wave-area apparent resistivity outside the wave area in frequency electromagnetic sounding.