The authors have obtained a regression equation about one measuring point using the least square algorithm which acts on the electromagnetic field created only by geological anomaly body instead of by mixed signal. The apparent resistivity which is just on the verge of the real electrical resistivity of the coal seam can be predicted appropriately by using this equation. At the same time, the change of the apparent resistivity curve after using least square algorithm is very stable. The value of this new apparent resistivity in the blind area can be used to figure out the value of the observed signal inside the blind zone. Then the response message of unmixed secondary field which is caused only by the geological anomaly body comes out, and this unmixed secondary field can be regarded as the filed separated from the mixed filed, and it brings a much better result at the early stage of advanced detection.