In this Paper, based on the data of earth resistivity and some measurements of resistivity of rock specimen, the author studied the influence of the thickness of Quarternary over-burden of soil on the normal variations and the precursors of the earth risistivity and the relation is also studied between the mechanical properties and changes of the earth resistivity of the rocks under the Quarternary over-burden soil. The observing station of the earth resistivity must be located at place where the porosity of rocks is low under the thinner Quarternary over-burden soil.