In this paper, the authors analyzed the characteristics of the horizontal electric dipole source electromagnetic expression of homogeneous half space, and then extended this expression to the bipolar source electromagnetic field. The relationships between CSAMT detecting depth and frequency, electric dipole moment, transceiver distance and the resistivity of the coating layer were studied based on bipole 1D simulation. The results show that, under the premise of guaranteed noise ratio, the factors that decide the maximum detecting depth of CSAMT method are corresponding frequencies when the detection enters the near region and transitional zone under the integrated action of transceiver distance and resistivity of the coating layer. The argument "the smaller the frequency, the larger the detecting depth' does not always hold water. When the detection enters the near region, the CSAMT method no longer has the detection capability. Statistics show that, when the electromagnetic field enters the transitional zone from the distant zone, the |kr| value is about 4.789; when the electromagnetic field enters the near zone from the transitional zone, the |kr| value is about 0.106. Then conclusions are drawn that the maximum detection depth of Cagniard apparent resistivity is about 0.3 times the transmitter receiver distance, and the maximum detection depth of full wave apparent resistivity is about 3.7 times the transmitter receiver distance. A case study has proved the above understanding.