Abstract: This paper discusses the definition and application of full-region apparent resistivity by using CSAMT method to explore the electric field in x-direction. Integral method is used to superimpose multiple electric dipoles to obtain electromagnetic field of bipolar source. The results calculated in the homogeneous half space show that the method is feasible. Computation of the apparent resistivity of the two or three layer sections and comparison between Cagniard apparent resistivity and the apparent resistivity in wave zone show the advantage of this method. The result calculated by the CSAMT can reflect the characteristics of the electrical section better through using the CSAMT method defined by the electric dipole to explore electric field in x direction. This method has practical value for the deep exploration, simplifying the field work, reducing data collection in field and solving the near field source problem in measuring Cagniard apparent resistivity.