Apparent resistivity has been one of the important branches of seismic geophysical field observation in China,which is applied to explore the process of earthquake preparation by marking the variation characteristics and spatial distribution of medium resistivity with time in a certain area. Through over 50 years of observation,we have recorded a series of the medium-term and short-term anomalies before about one hundred of M_S5.0～8.0 earthquakes and the impending earthquake changes before some strong earthquakes,as well as the recovery processes after the earthquakes. Resistivity is an important physical parameter of underground media,which is related to resistivity of both rock-soil and fluid,crack rate,crack structure and fluid saturation in geotechnical framework and fissures. Tectonic stress is an important factor leading to the change of resistivity. In previous related literatures the possible mechanism of resistivity variation before earthquakes have been discussed from rock physics experiments,electrical resistivity models and case studies. In this paper we introduce the possible mechanism of apparent resistivity anomalies associated with strong earthquakes along the line of “medium deformation-resistivity change”,combining with the results from rock physical experiment,resistivity model,and macroscopic phenomena of apparent resistivity anomalies around the epicenter. Seismic preparation process causes the deformation of the medium around the seismogenic fault,and the deformation degree decreases gradually along the seismogenic fault. In shallow surface medium under low confining pressure,the new micro-cracks will roughly expand along the direction of the maximum principal compressive stress,and then the resistivity of the medium will change. The apparent resistivity observed at the same station in different directions implies anisotropic changes related to the P axis orientation. The bigger the angle between the arrays and the P axis,the greater the amplitudes of the anomalies are. When the arrays are parallel to or sub-parallel to the P axis,the arrays have the smallest amplitudes of anomalies or the changes are even unnoticeable. Before a strong earthquake,apparent resistivity in the area of compressive deformation enhancement decreases,while that in the area of relative expansion such increases or the changes are not obviously.