Currently, the adoption of the ratio of electric field and magnetic field to calculate the Cagniard resistivity is widely used in traditional CSAMT exploration. However, an analysis of the electromagnetic field components and the apparent resistivity definition method in homogeneous half space and layered media indicates that the apparent resistivity defined by electric field E_x component can improve the problem of the near zone distortion of ratio apparent resistivity. Moreover, the advantage of apparent resistivity defined E_x component has been further verified by the measured field data. Therefore it's not necessary to rigidly stick to using the ratio apparent resistivity in the field CSAMT data acquisition, and the acquisition mode flexibility can be adjusted according to actual conditions so as to improve work quality and efficiency.