Abstract: High density resistivity method is a kind of exploration which regards the electricity difference of the rock and mineral as the foundation,by observation and study of the distributing regulation of underground stable electric field to resolve the mineral resources,environment and engineering geology problem,and by probing into the variety of field parameter to obtain the partial difference in electricity and to understand the measured target.This method can determine the overcast thickness by making use of the electricity difference between overcast layer and lower datum rock,which is one of the most valid methods for the high-efficiency,deep and accurate ground electricity section imaging.This article explained how to measure overcast thickness,method and the process of data processing and analyzing,and applied it to actual engineering.The actual application indicated that this method can get good result of measuring the overcast thickness,and is of lowcost,high-efficiency and simple characters,therefore it has the certain value in engineering application.