Considering the fact that there is only one pair of fixed potential poles in one direction used to monitor the resistiv-ity changes under the observatory station which may indicate process of earthquake preparation, based on the pre-condition that the resistivity structure is 1-dimensional under the observatory station, and by way of equivalent model put forward by the authors, this paper suggests a kind of method trying to separate the resistivity changes in deep layers from that of the shallow layer. Analysis shows that, to obtain the same measured data with the same array, any a multi-layered geo-electrical structure can be equivalently taken as a two-layered one, we call it equiva-lent model. In this paper, the resistivity and thickness of the upper layer of the equivalent model are equal to that of the first layer of the original one, the resistivity of the lower layer of the equivalent model is obtained with calcula-tion according to the requirement of equivalence, its value is relevant with the resistivities and thicknesses of all layer of the original model except for the first one, so the resistivity change of the equivalent layer can indicate the synthetic effect of variation of all layers of the original model except for the first one. The primary research results show that this method can work well. Considering the fact that there is only one pair of fixed potential poles in one direction used to monitor the resistiv- ity changes under the observatory station which may indicate the process of earthquake preparation, based on the pre-condition that the resistivity structure is 1-dimensional under the observatory station, and by way of equivalent model put forward by the authors, this paper suggests a kind of method trying to separate the resistivity changes in in deep layers from that of the shallow layer. with this same array, any a multi-layered geo-electrical structure can be equivalently taken as a two-layered one, we call it equiva lent model. In this paper, the resistivity and thickness of the upper layer of the equivalent model are equal to that of the first layer of the original one, the resistivity of the lower layer of the equivalent model is obtained with calcula tion according to the requirement of equivalence, its value is r elevant with the resistivities and thicknesses of all layers of the original model except for the first one, so the resistivity change of the equivalent layer can indicate the synthetic effect of variation of all layers of the original model except for the first one. results show that this method can work well.