Based on a study of the change in the resistivity for voxel in homogeneous half-space, this paper describes the basic principles of the response function. Focusing on the symmetric four-pole sounding, this paper also deals with the basic idea of equivalent-depth-mode electrode sequence method. With a practical example, the advantages of this method are discussed in the aspects of optimizing the sequence of electrodes, increasing the density of target depth information, and improving the capability of depth resolution. Based on the inversion results, this paper analyzes the feasibility of the estimation of the depth of the anomalous body by using the equivalent-depth-mode electrode sequence method.