An incomplete Cholesky factorization is used to preconditioning large sparse coefficient matrix in 3-D geoelectric field forward modelling. Coefficient matrix of the new system is an approximate unit matrix. Preconditioned Lanczos algorithm improves stability and convergence of iterative procedures. In the algorithm, as iterative numbers are smaller than dimensions of coefficient matrix, approximation of the exact solution is obtained. Preconditioned Lanczos algorithm is effective in 3-D geoelectric field forward modeling. It makes a foundaiton for 3-D geoelectric inversion.