Abstract: For the large sparse linear equations: A _x=_b, which are formed from the finite difference method used to solve the 3-D forward problem of geoelectrical field,in general,the computational efficiency with direct method is quite slow.In this paper,from the view point of the characteristics of the incomplete Cholesky decomposition of matrixX A and its eigenvalue,the internal cause of greatly increased 3-D resistivity forward speed using the incomplete Cholesky conjugate gradient (ICCG) iteration technique is explained.Introducing the row-indexed sparse storage mode to store matrix A , the internal storage demand is greatly decreased.