Abstract: Generally, due to the characteristic with nonuniformity of the weathering action in weathered rock site, Chinese standard "Code for investigation of geotechnical engineering (GB 50021-2001)" stipulates sternly the space of the exploratory hole in this kind of sites, bringing on large numbers of the exploratory holes. In this study, analyzing the feasibility and the necessity of multi-electrode electric method while carrying out geotechnical investigation in this kind of sites and using the example of the geotechnical investigation for a refinery site in Algeria, the optimization of multi-electrode resistivity method on the geotechnical investigation scheme for the large-scale weathering rock mass site is explained.