Abstract: To study the electromagnetic scattering of targets with the background of ocean, the program to choose the number of complex images when calculate Sommerfeld Integrals of the half-space problem with discrete complex image method (DCIM) applied is analyzed, which increases the efficiency of the subsequent Method of Moment (MoM). Multi-level DCIM is employed in the paper to get higher accuracy and robustness, and then in definite spatial-domain range and dielectric parameters, the minimum numbers of complex images to calculate the three typical kinds of Sommerfeld Integrals with same precision are presented.Therefore, the time for filling impedance matrix is minimized. It is shown that the reduction of complex images leads to an efficient MoM solution.