Abstract: This paper presents a dissection method with the double net system in the finite element method for 3-D numerical modeling of resistivity sounding.By the computation of several typical models using 3-D FEM with homogeneous conductivity and potential interpolated with a trilinear function in each block, we gave the distribution rules of true and false approximate resistivity anomaly with symmetrical four electrodes, analyzed the cause of true and false anomaly, and studied the relation both the general shape of the true anomaly and the position of geological body which produced true anomaly.